netfilter: ctnetlink: add and use a helper for mark parsing

ctnetlink dumps can be filtered based on the connmark.

Prepare for status bit filtering by using a named structure and by
moving the mark parsing code to a helper.

Else ctnetlink_alloc_filter size grows a bit too big for my taste
when status handling is added.

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