Adding function to calculate difference between two addresses.

Change-Id: If64d4b5f342f063dd0394c7bcc8a2389a63fe702
diff --git a/src/api.c b/src/api.c
index c9246c6..60954c3 100644
--- a/src/api.c
+++ b/src/api.c
@@ -1177,7 +1177,7 @@
 	 *       invalidation.
 	 */
 	void *ptr = mm_identity_map(begin, end, MM_MODE_W, ppool);
-	size_t size = pa_addr(end) - pa_addr(begin);
+	size_t size = pa_difference(begin, end);
 
 	if (!ptr) {
 		/* TODO: partial defrag of failed range. */