netfilter: nf_tables: don't drop IPv6 packets that cannot parse transport

This is overly conservative and not flexible at all, so better let them
go through and let the filtering policy decide what to do with them. We
use skb_header_pointer() all over the place so we would just fail to
match when trying to access fields from malformed traffic.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>