We need to re-check sqd->thread after we've dropped the lock. Pin
the sqd before doing the lockdep lock dance, and check if the thread
is alive after that. It's either NULL or alive, as the SQPOLL thread
cannot exit without holding the same sqd->lock.

Reported-and-tested-by: syzbot+337de45f13a4fd54d708@syzkaller.appspotmail.com
Fixes: fa84693b ("io_uring: ensure IORING_REGISTER_IOWQ_MAX_WORKERS works with SQPOLL")
Signed-off-by: Jens Axboe <axboe@kernel.dk>