netfilter: nf_queue: add NFQA_SKB_CSUM_NOTVERIFIED info flag

The common case is that TCP/IP checksums have already been
verified, e.g. by hardware (rx checksum offload), or conntrack.

Userspace can use this flag to determine when the checksum
has not been validated yet.

If the flag is set, this doesn't necessarily mean that the packet has
an invalid checksum, e.g. if NIC doesn't support rx checksum.

Userspace that sucessfully enabled NFQA_CFG_F_GSO queue feature flag can
infer that IP/TCP checksum has already been validated if either the
SKB_INFO attribute is not present or the NFQA_SKB_CSUM_NOTVERIFIED
flag is unset.

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