Add mtodo(m, o) macro taking an additional offset into the mbuf data section.

Sponsored by:	The FreeBSD Foundation

sys/sys/mbuf.h

@@ -67,8 +67,10 @@
  * type:
  *
  * mtod(m, t)	-- Convert mbuf pointer to data pointer of correct type.
+ * mtodo(m, o) -- Same as above but with offset 'o' into data.
  */
 #define	mtod(m, t)	((t)((m)->m_data))
+#define	mtodo(m, o)	((void *)(((m)->m_data) + (o)))
 
 /*
  * Argument structure passed to UMA routines during mbuf and packet