gtp: fix fragmentation needed check with gso

Call skb_gso_validate_network_len() to check if packet is over PMTU.

Fixes: 459aa660

 ("gtp: add initial driver for datapath of GPRS Tunneling Protocol (GTP-U)")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>