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/drivers/net/s2io.c b/drivers/net/s2io.c
index f4d80f9..043850b 100644
--- a/drivers/net/s2io.c
+++ b/drivers/net/s2io.c
@@ -4109,7 +4109,7 @@
 			struct tcphdr *th;
 			ip = ip_hdr(skb);
 
-			if ((ip->frag_off & htons(IP_OFFSET|IP_MF)) == 0) {
+			if (!ip_is_fragment(ip)) {
 				th = (struct tcphdr *)(((unsigned char *)ip) +
 						       ip->ihl*4);