Replace incorrect local OFFSET_OF macro with the correct and generic

offsetof macro.

sys/netinet/ip_dummynet.c

@@ -1696,7 +1696,7 @@ config_pipe(struct dn_pipe *p)
 			 */
 			pipe->idle_heap.size = pipe->idle_heap.elements = 0;
 			pipe->idle_heap.offset =
-			    OFFSET_OF(struct dn_flow_queue, heap_pos);
+			    offsetof(struct dn_flow_queue, heap_pos);
 		} else
 			/* Flush accumulated credit for all queues. */
 			for (i = 0; i <= pipe->fs.rq_size; i++)

sys/netinet/ip_dummynet.h

@@ -70,11 +70,6 @@ typedef u_int64_t dn_key ;      /* sorting key */
  * virtual time wraps every 15 days.
  */
 
-/*
- * The OFFSET_OF macro is used to return the offset of a field within
- * a structure. It is used by the heap management routines.
- */
-#define OFFSET_OF(type, field) ((int)&( ((type *)0)->field) )
 
 /*
  * The maximum hash table size for queues.  This value must be a power