conntrack: mnl: fix parsing payload len

Substract the netlink + nfnetlink headers to pass the payload length
to nfct_payload_parse().

Signed-off-by: Ken-ichirou MATSUZAWA <chamas@h4.dion.ne.jp>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
src: fix documentation regarding nfct_catch() and nfexp_catch()

Stefan reported that the *_catch() functions documentation was imprecise
on some aspects.

Reported-by: Stefan Nicolae Stancu <Stefan.Stancu@cern.ch>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>