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_output.c b/net/ipv4/ip_output.c
index a8024ea..167da8b 100644
--- a/net/ipv4/ip_output.c
+++ b/net/ipv4/ip_output.c
@@ -489,7 +489,7 @@
 
 		if (first_len - hlen > mtu ||
 		    ((first_len - hlen) & 7) ||
-		    (iph->frag_off & htons(IP_MF|IP_OFFSET)) ||
+		    ip_is_fragment(iph) ||
 		    skb_cloned(skb))
 			goto slow_path;