*: use array_size() helper macro

Use the array_size() helper macro.  Replaces several instances of local
macros with the same definition.

Reviewed-by: Scott Feldman <sfeldma@cumulusnetworks.com>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/lib/buffer.c b/lib/buffer.c
index f19a9e0..45e2e1c 100644
--- a/lib/buffer.c
+++ b/lib/buffer.c
@@ -262,7 +262,7 @@
   /* For erase and more data add two to b's buffer_data count.*/
   if (b->head->next == NULL)
     {
-      iov_alloc = sizeof(small_iov)/sizeof(small_iov[0]);
+      iov_alloc = array_size(small_iov);
       iov = small_iov;
     }
   else