netfilter: ctnetlink: use dump structure instead of raw args

netlink_dump structure has a union of 'long args[6]' and a context
buffer as scratch space.

Convert ctnetlink to use a structure, its easier to read than the
raw 'args' usage which comes with no type checks and no readable names.

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