io_uring: set task_work notify method at init time

While doing so, switch SQPOLL to TWA_SIGNAL_NO_IPI as well, as that
just does a task wakeup and then we can remove the special wakeup we
have in task_work_add.

Reviewed-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/20220426014904.60384-5-axboe@kernel.dk


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