dm writecache: add unlikely for returned value of rb_next/prev

In functions writecache_discard() and writecache_find_entry() there is a
high probablity that the pointer of structure rb_node won't equal NULL.
Add unlikely for the pointer node NULL.
Signed-off-by: Huaisheng Ye <yehs1@lenovo.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-writecache.c

@@ -571,7 +571,7 @@ static struct wc_entry *writecache_find_entry(struct dm_writecache *wc,
 			node = rb_prev(&e->rb_node);
 		else
 			node = rb_next(&e->rb_node);
-		if (!node)
+		if (unlikely(!node))
 			return e;
 		e2 = container_of(node, struct wc_entry, rb_node);
 		if (read_original_sector(wc, e2) != block)
@@ -804,7 +804,7 @@ static void writecache_discard(struct dm_writecache *wc, sector_t start, sector_
 			writecache_free_entry(wc, e);
 		}
 
-		if (!node)
+		if (unlikely(!node))
 			break;
 
 		e = container_of(node, struct wc_entry, rb_node);