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: 459aa660

 ("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>