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/sch_choke.c b/net/sched/sch_choke.c
index 06afbae..3422b25 100644
--- a/net/sched/sch_choke.c
+++ b/net/sched/sch_choke.c
@@ -181,7 +181,7 @@
 		    ip1->saddr != ip2->saddr || ip1->daddr != ip2->daddr)
 			return false;
 
-		if ((ip1->frag_off | ip2->frag_off) & htons(IP_MF | IP_OFFSET))
+		if (ip_is_fragment(ip1) | ip_is_fragment(ip2))
 			ip_proto = 0;
 		off1 += ip1->ihl * 4;
 		off2 += ip2->ihl * 4;