malloc: set pad to 0 on free

The pad value is not used unless element is in pad state, but it
will show up in heap dumps and may be confusing.

Signed-off-by: Anatoly Burakov <anatoly.burakov@intel.com>

lib/librte_eal/common/malloc_elem.c

@@ -445,6 +445,8 @@ malloc_elem_free(struct malloc_elem *elem)
 
 	malloc_elem_free_list_insert(elem);
 
+	elem->pad = 0;
+
 	/* decrease heap's count of allocated elements */
 	elem->heap->alloc_count--;