9p: trans_fd/p9_conn_cancel: drop client lock earlier

syzbot reported a double-lock here and we no longer need this
lock after requests have been moved off to local list:
just drop the lock earlier.

Link: https://lkml.kernel.org/r/20220904064028.1305220-1-asmadeus@codewreck.org
Reported-by:  <syzbot+50f7e8d06c3768dd97f3@syzkaller.appspotmail.com>
Signed-off-by: Dominique Martinet <asmadeus@codewreck.org>
Tested-by: Schspa Shi <schspa@gmail.com>