io_uring: improve sqo stop

Set IO_SQ_THREAD_SHOULD_STOP before taking sqd lock, so the sqpoll task
sees earlier. Not a problem, it will stop eventually. Also check
invariant that it's stopped only once.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/653b24ee93843a50ff65a45847d9138f5adb76d7.1618101759.git.asml.silence@gmail.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>