netfilter: nft_payload: simplify vlan header handling

If the offset is within the ethernet + vlan header size boundary, then
rebuild the ethernet + vlan header and use it to copy the bytes to the
register. Otherwise, subtract the vlan header size from the offset and
fall back to use skb_copy_bits().

There is one corner case though: If the offset plus the length of the
payload instruction goes over the ethernet + vlan header boundary, then,
fetch as many bytes as possible from the rebuilt ethernet + vlan header
and fall back to copy the remaining bytes through skb_copy_bits().

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Acked-by: Florian Westphal <fw@strlen.de>