netfilter: nfnetlink_queue: add skb info attribute

Once we allow userspace to receive gso/gro packets, userspace
needs to be able to determine when checksums appear to be
broken, but are not.

NFQA_SKB_CSUMNOTREADY means 'checksums will be fixed in kernel
later, pretend they are ok'.

NFQA_SKB_GSO could be used for statistics, or to determine when
packet size exceeds mtu.

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