netfilter: nf_tables: Always allocate nft_rule_dump_ctx

It will move into struct netlink_callback's scratch area later, just put
nf_tables_dump_rules_start in shape to reduce churn later.

Suggested-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Florian Westphal <fw@strlen.de>