netfilter: nfnetlink_queue: avoid expensive gso segmentation and checksum fixup

Userspace can now indicate that it can cope with larger-than-mtu sized
packets and packets that have invalid ipv4/tcp checksums.

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