Clean up to kernel code, Added `used' field to capabilities

With the addition of used field, capability structure can be
used for resource accounting over the capability.

tasks/mm0/include/capability.h

@@ -30,6 +30,8 @@ struct capability {
 	unsigned long start;	/* Resource start value */
 	unsigned long end;	/* Resource end value */
 	unsigned long size;	/* Resource size */
+
+	unsigned long used;	/* Resource used size */
 };