9p/fd: fix issue of list_del corruption in p9_fd_cancel()

Syz reported the following issue:
kernel BUG at lib/list_debug.c:53!
invalid opcode: 0000 [#1] PREEMPT SMP KASAN
RIP: 0010:__list_del_entry_valid.cold+0x5c/0x72
Call Trace:
<TASK>
p9_fd_cancel+0xb1/0x270
p9_client_rpc+0x8ea/0xba0
p9_client_create+0x9c0/0xed0
v9fs_session_init+0x1e0/0x1620
v9fs_mount+0xba/0xb80
legacy_get_tree+0x103/0x200
vfs_get_tree+0x89/0x2d0
path_mount+0x4c0/0x1ac0
__x64_sys_mount+0x33b/0x430
do_syscall_64+0x35/0x80
entry_SYSCALL_64_after_hwframe+0x46/0xb0
</TASK>

The process is as follows:
Thread A:                       Thread B:
p9_poll_workfn()                p9_client_create()
...                                 ...
    p9_conn_cancel()                p9_fd_cancel()
        list_del()                      ...
        ...                             list_del()  //list_del
                                                      corruption
There is no lock protection when deleting list in p9_conn_cancel(). After
deleting list in Thread A, thread B will delete the same list again. It
will cause issue of list_del corruption.

Setting req->status to REQ_STATUS_ERROR under lock prevents other
cleanup paths from trying to manipulate req_list.
The other thread can safely check req->status because it still holds a
reference to req at this point.

Link: https://lkml.kernel.org/r/20221110122606.383352-1-shaozhengchao@huawei.com
Fixes: 52f1c45d

 ("9p: trans_fd/p9_conn_cancel: drop client lock earlier")
Reported-by:  <syzbot+9b69b8d10ab4a7d88056@syzkaller.appspotmail.com>
Signed-off-by: Zhengchao Shao <shaozhengchao@huawei.com>
[Dominique: add description of the fix in commit message]
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>