io_uring: add IORING_SETUP_SCHED_SUBMIT support for SQPOLL

io_uring: add IORING_SETUP_SCHED_SUBMIT support for SQPOLL

For SQPOLL, this enables automatic submission of requests queued for the
SQPOLL thread when the application task schedules out, regardless of
whether or not the SQPOLL is actively running or currently sleeping. If
it's sleeping, it'll get woken automatically when the issuing task goes
to sleep.

If IORING_SETUP_SCHED_SUBMIT is set with SQPOLL, the
IORING_SQ_NEED_WAKEUP flag is no longer being set when the SQPOLL thread
goes to sleep. This isn't necessary anymore, as the SQPOLL thread will
get woken automatically when the issuing task goes to sleep or schedules
out.

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