【标题描述】hisi_sas驱动加载时初始化debugfs，申请内存失败后，在驱动卸载或设备移除时会触发空指针异常
【环境信息】
硬件信息：
1） 鲲鹏920
软件信息：
[root@localhost 0000:b4:02.0]# cat /etc/euleros-latest
eulerversion=EulerOS_Server_V200R008C00SPC300B630
compiletime=2019-12-27-10-58-38
kernelversion=4.19.36-vhulk1907.1.0.h619
[root@localhost 0000:b4:02.0]# uname -a
Linux localhost.localdomain 4.19.36-vhulk1907.1.0.h619.eulerosv2r8.aarch64 #1 SMP Mon Jul 22 00:00:00 UTC 2019 aarch64 aarch64 aarch64 GNU/Linux
【问题复现步骤】
问题前提：系统内存资源不足。
具体操作步骤： 对控制器进行加卸载测试：
echo 1 > /sys/bus/pci/devices/0000:74:02.0/remove
echo 1 > /sys/bus/pci/rescan
出现概率：80%
【预期结果】
设备能正常完成加卸载。
【实际结果】
出现空指针异常
【解决方案】
debugfs_dir资源释放后置NULL，防止第2次去释放时，触发空指针异常。
【附件信息】
日志：
localhost:/verification/ko # echo 1 > /sys/bus/pci/rescan
[29405.891961] pci 0000:74:02.0: [19e5:a230] type 00 class 0x010700
[29405.897969] pci 0000:74:02.0: reg 0x24: [mem 0xa2000000-0xa2007fff]
[29405.904352] pci 0000:74:02.0: BAR 5: assigned [mem 0xa2000000-0xa2007fff]
[29405.911919] iommu: Adding device 0000:74:02.0 to group 12
[29405.934500] kworker/0:2: page allocation failure: order:7, mode:0x6040c0(GFP_KERNEL|__GFP_COMP), nodemask=(null)
[29405.944670] kworker/0:2 cpuset=/ mems_allowed=0
[29405.949197] CPU: 0 PID: 337 Comm: kworker/0:2 Tainted: G W OE 4.19.36-vhulk1907.1.0.h619.eulerosv2r8.aarch64 #1
[29405.960310] Hardware name: Huawei Technologies Co., Ltd. EVBCS/EVBCS, BIOS CS B200 1P TA 01/15/2022
[29405.969343] Workqueue: events work_for_cpu_fn
[29405.973686] Call trace:
[29405.976120] dump_backtrace+0x0/0x198
[29405.979768] show_stack+0x24/0x30
[29405.983069] dump_stack+0xa4/0xcc
[29405.986372] warn_alloc+0x10c/0x190
[29405.989846] __alloc_pages_nodemask+0xc78/0xd38
[29405.994363] kmalloc_large_node+0x6c/0x90
[29405.998359] __kmalloc_node_track_caller+0x268/0x330
[29406.003310] devm_kmalloc+0x44/0xb0
[29406.006788] hisi_sas_debugfs_init+0x370/0x480 [hisi_sas_main]
[29406.012608] hisi_sas_v3_probe+0x53c/0x590 [hisi_sas_v3_hw]
[29406.018168] local_pci_probe+0x44/0xa8
[29406.021903] work_for_cpu_fn+0x20/0x30
[29406.025638] process_one_work+0x1b4/0x3f8
[29406.029633] worker_thread+0x210/0x470
[29406.033368] kthread+0x134/0x138
[29406.036583] ret_from_fork+0x10/0x18
[29406.040151] warn_alloc_show_mem: 1 callbacks suppressed

localhost:/verification/ko # echo 1 > /sys/bus/pci/devices/0000:74:02.0/remove
[29413.635882] Unable to handle kernel NULL pointer dereference at virtual address 00000000000000a0
[29413.644656] Mem abort info:
[29413.647435] ESR = 0x96000004
[29413.650481] Exception class = DABT (current EL), IL = 32 bits
[29413.656389] SET = 0, FnV = 0
[29413.659429] EA = 0, S1PTW = 0
[29413.662559] Data abort info:
[29413.665429] ISV = 0, ISS = 0x00000004
[29413.669254] CM = 0, WnR = 0
[29413.672210] user pgtable: 4k pages, 48-bit VAs, pgdp = 000000006840720c
[29413.678813] [00000000000000a0] pgd=0000000000000000
[29413.683681] Internal error: Oops: 96000004 [#1] SMP
[29413.688545] Process bash (pid: 4708, stack limit = 0x0000000088630e8c)
[29413.695058] CPU: 27 PID: 4708 Comm: bash Tainted: G W OE 4.19.36-vhulk1907.1.0.h619.eulerosv2r8.aarch64 #1
[29413.705737] Hardware name: Huawei Technologies Co., Ltd. EVBCS/EVBCS, BIOS CS B200 1P TA 01/15/2022
[29413.714767] pstate: 80400009 (Nzcv daif +PAN -UAO)
[29413.719545] pc : down_write+0x24/0x70
[29413.723193] lr : down_write+0x1c/0x70
[29413.726840] sp : ffff00001df6ba30
[29413.730141] x29: ffff00001df6ba30 x28: ffff802fb835dd00
[29413.735439] x27: ffff802a89140000 x26: 0000000000000000
[29413.740737] x25: ffff802f5201ca90 x24: ffff802f5201ca90
[29413.746035] x23: ffff00000986dc00 x22: ffff00000986dbf0
[29413.751333] x21: 0000000000000000 x20: ffff802f5201cb30
[29413.756631] x19: 00000000000000a0 x18: ffff802fb887d810
[29413.761929] x17: 0000000000000000 x16: 0000000000000000
[29413.767226] x15: 0000000000000000 x14: ffff8023e23f2500
[29413.772524] x13: ffff8023e23f2480 x12: 0000000000000000
[29413.777822] x11: ffff8023e23f24a8 x10: ffff802fb4fb8dd0
[29413.783120] x9 : 000000000000000c x8 : 0000000000000011
[29413.788417] x7 : ffff7e00a1ed3300 x6 : 0000000000000001
[29413.793715] x5 : 0000000000000001 x4 : ffff802f59dac238
[29413.799012] x3 : ffff802f51a9a19c x2 : 0000000000000000
[29413.804310] x1 : 00000000000000a0 x0 : ffffffff00000001
[29413.809607] Call trace:
[29413.812040] down_write+0x24/0x70
[29413.815343] debugfs_remove_recursive+0x5c/0x1b0
[29413.819950] hisi_sas_debugfs_exit+0x24/0x30 [hisi_sas_main]
[29413.825596] hisi_sas_v3_remove+0x34/0x150 [hisi_sas_v3_hw]
[29413.831155] pci_device_remove+0x48/0xd8
[29413.835063] device_release_driver_internal+0x1b4/0x250
[29413.840273] device_release_driver+0x28/0x38
[29413.844530] pci_stop_bus_device+0x84/0xb8
[29413.848612] pci_stop_and_remove_bus_device_locked+0x24/0x40
[29413.854257] remove_store+0xfc/0x140
[29413.857818] dev_attr_store+0x44/0x60
[29413.861466] sysfs_kf_write+0x58/0x80
[29413.865114] kernfs_fop_write+0xe8/0x1f0
[29413.869023] __vfs_write+0x60/0x190
[29413.872497] vfs_write+0xac/0x1c0
[29413.875798] ksys_write+0x6c/0xd8
[29413.879099] __arm64_sys_write+0x24/0x30
[29413.883008] el0_svc_common+0x78/0x130
[29413.886743] el0_svc_handler+0x38/0x78
[29413.890478] el0_svc+0x8/0xc