io_uring/sqpoll: don't attach to a thread that is canceling

io_sq_offload_create() refuses to attach a ring to an SQPOLL thread
that has already gone, by checking sqd->thread under sqd->lock. Since
the thread drops the lock while io_uring_cancel_generic() waits for its
requests, that check can run while the thread is on its way out and
still see it. The new ring is then added to ctx_list, io_uring_setup()
succeeds, the thread finishes canceling and exits without ever having
looked at the ring, and io_uring_enter() fails with -EOWNERDEAD.

Only a thread that exits on a fatal signal can be caught like this, as
the stop bit is set once the last ring has let go of the sqd, when no fd
is left to attach through. A fatal signal is group wide and attaching
needs the same thread group, so the attacher is on its way out as well.
Still, make the check see it: set the stop bit before canceling,
whatever the reason for the exit, and test it when attaching.

Reported-by: Karl Mehltretter <kmehltretter@gmail.com>
Fixes: cb0306c52ffa ("io_uring/cancel: drop sqd->lock while SQPOLL waits")
Signed-off-by: Jens Axboe <axboe@kernel.dk>