netfilter: nf_tables: Don't allocate nft_rule_dump_ctx

Since struct netlink_callback::args is not used by rule dumpers anymore,
use it to hold nft_rule_dump_ctx. Add a build-time check to make sure it
won't ever exceed the available space.

Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Florian Westphal <fw@strlen.de>