gtp: reload GTPv1 header after pskb_may_pull()

The GTPv1 header flags indicate the presence of optional extensions
after this header. Refresh the pointer to the GTPv1 header as skb->head
might have be reallocated via pskb_may_pull().

Fixes: 459aa66 ("gtp: add initial driver for datapath of GPRS Tunneling Protocol (GTP-U)")
Reported-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/gtp.c

@@ -253,6 +253,8 @@ static int gtp1u_udp_encap_recv(struct gtp_dev *gtp, struct sk_buff *skb,
 	if (!pskb_may_pull(skb, hdrlen))
 		return -1;
 
+	gtp1 = (struct gtp1_header *)(skb->data + sizeof(struct udphdr));
+
 	rcu_read_lock();
 	pctx = gtp1_pdp_find(gtp, ntohl(gtp1->tid));
 	if (!pctx) {