io_uring: don't split out consume out of SQE get

Remove io_consume_sqe() and inline it back into io_get_sqe(). It
requires req dealloc on error, but in exchange we get cleaner
io_submit_sqes() and better locality for cached_sq_head.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>