net: hns3: minor optimization for ring_space

This patch optimizes the ring_space by calculating the
ring space without calling ring_dist.

Also ring_dist is only used by ring_space, so this patch
removes it when it is no longer used.
Signed-off-by: NYunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: NPeng Li <lipeng321@huawei.com>
Signed-off-by: NHuazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/hisilicon/hns3/hns3_enet.h

@@ -577,18 +577,13 @@ union l4_hdr_info {
 	unsigned char *hdr;
 };
 
-/* the distance between [begin, end) in a ring buffer
- * note: there is a unuse slot between the begin and the end
- */
-static inline int ring_dist(struct hns3_enet_ring *ring, int begin, int end)
-{
-	return (end - begin + ring->desc_num) % ring->desc_num;
-}
-
 static inline int ring_space(struct hns3_enet_ring *ring)
 {
-	return ring->desc_num -
-		ring_dist(ring, ring->next_to_clean, ring->next_to_use) - 1;
+	int begin = ring->next_to_clean;
+	int end = ring->next_to_use;
+
+	return ((end >= begin) ? (ring->desc_num - end + begin) :
+			(begin - end)) - 1;
 }
 
 static inline int is_ring_empty(struct hns3_enet_ring *ring)