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

[ Upstream commit 70c261d500951cf3ea0fcf32651aab9a65a91471 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>