netfilter: nfnetlink_queue: unbreak SCTP traffic

when packet is enqueued with nfqueue and GSO is enabled, checksum
calculation has to take into account the protocol, as SCTP uses a
32 bits CRC checksum.

Enter skb_gso_segment() path in case of SCTP GSO packets because
skb_zerocopy() does not support for GSO_BY_FRAGS.

Joint work with Pablo.

Signed-off-by: Antonio Ojea <aojea@google.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>