netfilter: nf_tables_ipv6: consider network offset in netdev/egress validation

From netdev/egress, skb->len can include the ethernet header, therefore,
subtract network offset from skb->len when validating IPv6 packet length.

Fixes: 42df6e1d ("netfilter: Introduce egress hook")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>