io_uring: add likely/unlikely in io_get_sqring()

The number of SQEs to submit is specified by a user, so io_get_sqring()
in most of the cases succeeds. Hint compilers about that.

Checking ASM genereted by gcc 9.2.0 for x64, there is one branch
misprediction.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -3129,11 +3129,11 @@ static bool io_get_sqring(struct io_ring_ctx *ctx, struct sqe_submit *s)
 	 */
 	head = ctx->cached_sq_head;
 	/* make sure SQ entry isn't read before tail */
-	if (head == smp_load_acquire(&rings->sq.tail))
+	if (unlikely(head == smp_load_acquire(&rings->sq.tail)))
 		return false;
 
 	head = READ_ONCE(sq_array[head & ctx->sq_mask]);
-	if (head < ctx->sq_entries) {
+	if (likely(head < ctx->sq_entries)) {
 		s->ring_file = NULL;
 		s->sqe = &ctx->sq_sqes[head];
 		s->sequence = ctx->cached_sq_head;