ip: introduce ip_is_fragment helper inline function

There are enough instances of this:

    iph->frag_off & htons(IP_MF | IP_OFFSET)

that a helper function is probably warranted.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/net/sched/cls_rsvp.h b/net/sched/cls_rsvp.h
index 402c44b..ed691b1 100644
--- a/net/sched/cls_rsvp.h
+++ b/net/sched/cls_rsvp.h
@@ -167,7 +167,7 @@
 	dst = &nhptr->daddr;
 	protocol = nhptr->protocol;
 	xprt = ((u8 *)nhptr) + (nhptr->ihl<<2);
-	if (nhptr->frag_off & htons(IP_MF | IP_OFFSET))
+	if (ip_is_fragment(nhptr))
 		return -1;
 #endif