netfilter: nf_tables: add NFTA_RULE_POSITION_ID to nla_policy

Fixes: 75dd48e2 ("netfilter: nf_tables: Support RULE_ID reference in new rule")
Reported-by: Cong Wang <xiyou.wangcong@gmail.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Acked-by: Phil Sutter <phil@nwl.cc>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>