io_uring: revert "io_uring: optimize submit_and_wait API"

There are cases where it isn't always safe to block for submission,
even if the caller asked to wait for events as well. Revert the
previous optimization of doing that.

This reverts two commits:

bf7ec93c
c5766668

Fixes: c5766668 ("io_uring: optimize submit_and_wait API")
Signed-off-by: Jens Axboe <axboe@kernel.dk>