GSO: Reload iph after pskb_may_pull

As it may get stale and lead to use after free.

Acked-by: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Alexander Duyck <aduyck@mirantis.com>
Cc: Andrey Konovalov <andreyknvl@google.com>
Fixes: cbc53e0 ("GSO: Add GSO type for fixed IPv4 ID")
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/af_inet.c

@@ -1233,7 +1233,7 @@ struct sk_buff *inet_gso_segment(struct sk_buff *skb,
 		fixedid = !!(skb_shinfo(skb)->gso_type & SKB_GSO_TCP_FIXEDID);
 
 		/* fixed ID is invalid if DF bit is not set */
-		if (fixedid && !(iph->frag_off & htons(IP_DF)))
+		if (fixedid && !(ip_hdr(skb)->frag_off & htons(IP_DF)))
 			goto out;
 	}