BUG/MINOR: ring: fix the size check in ring_make_from_area()

Fix the size check in ring_make_from_area() which is checking the size
of the pointer instead of the size of the structure.

No backport needed, 2.7 only.
diff --git a/src/ring.c b/src/ring.c
index 089a2fc..da987c5 100644
--- a/src/ring.c
+++ b/src/ring.c
@@ -85,7 +85,7 @@
 {
 	struct ring *ring = NULL;
 
-	if (size < sizeof(ring))
+	if (size < sizeof(*ring))
 		return NULL;
 
 	if (!area)