【OLK-5.10】mm/filemap: fix UAF in find_lock_entries

Release refcount after xas_set to fix UAF which may cause panic like this:
    
     page:ffffea000491fa40 refcount:1 mapcount:0 mapping:0000000000000000 index:0x1 pfn:0x1247e9
     head:ffffea000491fa00 order:3 compound_mapcount:0 compound_pincount:0
     memcg:ffff888104f91091
     flags: 0x2fffff80010200(slab|head|node=0|zone=2|lastcpupid=0x1fffff)
    ...
    page dumped because: VM_BUG_ON_PAGE(PageTail(page))
     ------------[ cut here ]------------
     kernel BUG at include/linux/page-flags.h:632!
     invalid opcode: 0000 [#1] SMP DEBUG_PAGEALLOC KASAN
     CPU: 1 PID: 7642 Comm: sh Not tainted 5.15.51-dirty #26
    ...
     Call Trace:
      <TASK>
      __invalidate_mapping_pages+0xe7/0x540
      drop_pagecache_sb+0x159/0x320
      iterate_supers+0x120/0x240
      drop_caches_sysctl_handler+0xaa/0xe0
      proc_sys_call_handler+0x2b4/0x480
      new_sync_write+0x3d6/0x5c0
      vfs_write+0x446/0x7a0
      ksys_write+0x105/0x210
      do_syscall_64+0x35/0x80
      entry_SYSCALL_64_after_hwframe+0x44/0xae
     RIP: 0033:0x7f52b5733130
    ...
    
    This problem has been fixed on mainline by patch 6b24ca4a1a8d ("mm: Use
    multi-index entries in the page cache") since it deletes the related code.

Fixes: 5c211ba29deb ("mm: add and use find_lock_entries")
    Signed-off-by: Liu Shixin <liushixin2@huawei.com>
    Acked-by: Matthew Wilcox (Oracle) <willy@infradead.org>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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