src/queue: only enter the kernel if both submit and need_enter are true

We just take the submit count, and then unconditionally just call
sq_ring_needs_enter() to ensure that flags are set appropriately. Only
set need_enter for the submit side if sq_ring_needs_enter() is true,
which may not be the case if SQPOLL is used.

This can avoid a system call if we're not waiting on any CQEs, and
the thread is already running.

Reported-by: Olivier Langlois <olivier@trillion01.com>
Link: #429 (comment)
Signed-off-by: Jens Axboe <axboe@kernel.dk>

src/queue.c

@@ -112,10 +112,8 @@ static int _io_uring_get_cqe(struct io_uring *ring, struct io_uring_cqe **cqe_pt
 			flags = IORING_ENTER_GETEVENTS | data->get_flags;
 			need_enter = true;
 		}
-		if (data->submit) {
-			sq_ring_needs_enter(ring, &flags);
+		if (data->submit && sq_ring_needs_enter(ring, &flags))
 			need_enter = true;
-		}
 		if (!need_enter)
 			break;