Rename completion helpers

We have io_uring_get_sqe() on the submission side, yet the completion
side is named _completion. Rename as follows:

io_uring_get_completion()	io_uring_peek_cqe()
iO_uring_wait_completion()	io_uring_wait_cqe()

This better tells the user what the _get variant does by calling it
_peek instead, and we move to using _cqe() as the postfix instead
of _completion.

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

test/io_uring_enter.c

@@ -146,9 +146,9 @@ reap_events(struct io_uring *ring, unsigned nr)
 	printf("Reaping %u I/Os\n", nr);
 	gettimeofday(&start, NULL);
 	while (left) {
-		ret = io_uring_wait_completion(ring, &cqe);
+		ret = io_uring_wait_cqe(ring, &cqe);
 		if (ret < 0) {
-			printf("io_uring_wait_completion returned %d\n", ret);
+			printf("io_uring_wait_cqe returned %d\n", ret);
 			printf("expected success\n");
 			exit(1);
 		}