netfilter: nfnetlink_queue: only add CAP_LEN attr when needed

CAP_LEN contains the size of the network packet we're queueing to
userspace, i.e. normally it is the same as the NFQA_PAYLOAD attribute len.

Include it only in the unlikely case when NFQA_PAYLOAD is truncated due
to copy_range limitations.

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