igb: Mask off check of frag_off as we only want fragment offset

We were incorrectly checking the entire frag_off field when we only wanted the
fragment offset.  As a result we were not pulling in TCP headers when the DNF
flag was set.

To correct that we will now check for frag off using the IP_OFFSET mask.
Signed-off-by: NAlexander Duyck <alexander.h.duyck@intel.com>
Tested-by: NAaron Brown <aaron.f.brown@intel.com>
Signed-off-by: NJeff Kirsher <jeffrey.t.kirsher@intel.com>

drivers/net/ethernet/intel/igb/igb_main.c

@@ -6472,7 +6472,7 @@ static unsigned int igb_get_headlen(unsigned char *data,
 			return hdr.network - data;
 
 		/* record next protocol if header is present */
-		if (!hdr.ipv4->frag_off)
+		if (!(hdr.ipv4->frag_off & htons(IP_OFFSET)))
 			nexthdr = hdr.ipv4->protocol;
 	} else if (protocol == __constant_htons(ETH_P_IPV6)) {
 		if ((hdr.network - data) > (max_len - sizeof(struct ipv6hdr)))