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/ip_input.c b/net/ipv4/ip_input.c
index c8f48ef..073a9b0 100644
--- a/net/ipv4/ip_input.c
+++ b/net/ipv4/ip_input.c
@@ -165,7 +165,7 @@
 		    (!sk->sk_bound_dev_if ||
 		     sk->sk_bound_dev_if == dev->ifindex) &&
 		    net_eq(sock_net(sk), dev_net(dev))) {
-			if (ip_hdr(skb)->frag_off & htons(IP_MF | IP_OFFSET)) {
+			if (ip_is_fragment(ip_hdr(skb))) {
 				if (ip_defrag(skb, IP_DEFRAG_CALL_RA_CHAIN))
 					return 1;
 			}
@@ -256,7 +256,7 @@
 	 *	Reassemble IP fragments.
 	 */
 
-	if (ip_hdr(skb)->frag_off & htons(IP_MF | IP_OFFSET)) {
+	if (ip_is_fragment(ip_hdr(skb))) {
 		if (ip_defrag(skb, IP_DEFRAG_LOCAL_DELIVER))
 			return 0;
 	}