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/ipv4/ipconfig.c b/net/ipv4/ipconfig.c
index ab7e554..ae93dd5 100644
--- a/net/ipv4/ipconfig.c
+++ b/net/ipv4/ipconfig.c
@@ -932,7 +932,7 @@
 		goto drop;
 
 	/* Fragments are not supported */
-	if (h->frag_off & htons(IP_OFFSET | IP_MF)) {
+	if (ip_is_fragment(h)) {
 		if (net_ratelimit())
 			printk(KERN_ERR "DHCP/BOOTP: Ignoring fragmented "
 			       "reply.\n");