Backport:fix several bugs, support clang build · Pull Request !162 · src-openEuler/open-iscsi

# PR主要描述信息

* 此PR基于[这个oEEP](https://gitee.com/openeuler/TC/blob/master/oEEP/oEEP-0003%20LLVM%E5%B9%B3%E8%A1%8C%E5%AE%87%E5%AE%99%E8%AE%A1%E5%88%92--%E5%9F%BA%E4%BA%8ELLVM%E6%8A%80%E6%9C%AF%E6%A0%88%E6%9E%84%E5%BB%BAoE%E8%BD%AF%E4%BB%B6%E5%8C%85.md)，旨在使得该软件包可以同时支持GCC和LLVM构建。
* 不影响原来的构建行为
* 上游PR：https://github.com/open-iscsi/open-iscsi/pull/478
* **问题一：**

```c
  [  102s] clang: error: -Wl,-z,relro: 'linker' input unused [-Werror,-Wunused-command-line-argument]
  [  102s] clang: error: -Wl,-z,now: 'linker' input unused [-Werror,-Wunused-command-line-argument]
  [  102s] clang: error: -lkmod: 'linker' input unused [-Werror,-Wunused-command-line-argument]
  ```

* **问题成因总结：**  spec文件中将链接器(linker)选项多余地传给了编译器
* **修改建议：**  在%if "%toolchain" \=\= "clang"判断控制条件下，将编译器与链接器选项分开，编译器选项给CFLAGS，链接器选项给LDFLAGS，使llvm能够构建，不影响gcc构建。

* **问题二：**

```c
  [   93s] In file included from io.c:41:
  [   93s] In file included from ./iface.h:23:
  [   93s] In file included from ../libopeniscsiusr/libopeniscsiusr/libopeniscsiusr.h:30:
  [   93s] ../libopeniscsiusr/libopeniscsiusr/libopeniscsiusr_common.h:75:8: error: attribute declaration must precede definition [-Werror,-Wignored-attributes]
  [   93s]    75 | struct __DLL_EXPORT iscsi_session;
  [   93s]       |        ^
  [   93s] ../libopeniscsiusr/libopeniscsiusr/libopeniscsiusr_common.h:64:38: note: expanded from macro '__DLL_EXPORT'
  [   93s]    64 | #define __DLL_EXPORT    __attribute__ ((visibility ("default")))
  [   93s]       |                                         ^
  [   93s] ./initiator.h:198:16: note: previous definition is here
  [   93s]   198 | typedef struct iscsi_session {
  [   93s]       |                ^
  ```

* **问题成因总结：** clang要求"attribute declaration must precede definition"即带有attribute的声明必须先于定义，在该问题情境中体现为头文件的顺序，\#include "iface.h"要先于\#include "initiator.h"
* **修改建议：** 在出现该问题的文件中，将\#include "iface.h"移动到\#include "initiator.h"之前

* **问题三：**

```c
  [   89s] In file included from session_info.c:19:
  [   89s] ./iscsid_req.h:21:9: error: 'ISCSID_REQ_H_' is used as a header guard here, followed by #define of a different macro [-Werror,-Wheader-guard]
  [   89s]    21 | #ifndef ISCSID_REQ_H_
  [   89s]       |         ^~~~~~~~~~~~~
  [   89s] ./iscsid_req.h:22:9: note: 'ISCSID_REQ_H' is defined here; did you mean 'ISCSID_REQ_H_'?
  [   89s]    22 | #define ISCSID_REQ_H
  [   89s]       |         ^~~~~~~~~~~~
  [   89s]       |         ISCSID_REQ_H_
  [   89s] 1 error generated.
  ```

* **问题成因总结：** 少了个下划线
* **修改建议：** 回合上游commit：[fix: add usr/iscsid_req.h missinig underline (#431) · hanqingwu/open-iscsi@2989724 (github.com)](https://github.com/hanqingwu/open-iscsi/commit/29897249ab77739f6b233677d761e24705b33f6a)

* **问题四：**

```c
  [   88s] mgmt_ipc.c:511:19: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]
  [   88s]   511 |         qtask->allocated = 1;
  [   88s]       |                          ^ ~
  [   88s] 1 error generated.
  ```

* **问题成因总结：**  这个错误是由于在代码中存在一个隐式截断问题。试图将一个 `int` 类型的值（这里是 `1`）赋给一个一位宽的位域（allocated），但由于位域的宽度为1位，这样的操作可能会导致值从`1`被截断为`-1`
* **修改建议：** 修改`int allocated : 1`为`unsigned int allocated : 1`

* **问题五：**

```c
  [   93s] discovery.c:698:6: error: variable 'num_targets' set but not used [-Werror,-Wunused-but-set-variable]
  [   93s]   698 |         int num_targets = 0;
  [   93s]       |             ^
  [   93s] 1 warning and 1 error generated.
  ```

* **问题成因总结：** num_targets变量在定义之后，只有自增操作，并没有被具体使用
* **修改建议：** 可以将这三处出现的地方（一处定义，两处自增）删除的

* **问题六：**

```c
  [   95s] In file included from discoveryd.c:31:
  [   95s] ./iface.h:61:29: error: declaration of 'struct iovec' will not be visible outside of this function [-Werror,-Wvisibility]
  [   95s]    61 |                                   int iface_all, struct iovec *iovs);
  [   95s]       |                                                         ^
  [   95s] 1 error generated.
  ```

* **问题成因总结：** struct iovec的定义在系统库 <sys/uio.h>中，而iface.h中既没有对struct iovec的定义，也没有包含该头文件
* **修改建议：** 在iface.h头文件中#include<sys/uio.h>

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