9p/trans_fd: Fix concurrency del of req_list in p9_fd_cancelled/p9_read_work

p9_read_work and p9_fd_cancelled may be called concurrently.
In some cases, req->req_list may be deleted by both p9_read_work
and p9_fd_cancelled.

We can fix it by ignoring replies associated with a cancelled
request and ignoring cancelled request if message has been received
before lock.

Link: http://lkml.kernel.org/r/20200612090833.36149-1-wanghai38@huawei.com
Fixes: 60ff779c

 ("9p: client: remove unused code and any reference to "cancelled" function")
Cc: <stable@vger.kernel.org> # v3.12+
Reported-by:  <syzbot+77a25acfa0382e06ab23@syzkaller.appspotmail.com>
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>