netfilter: nf_tables: use NLA_POLICY_MASK to test for valid flag options

nf_tables relies on manual test of netlink attributes coming from userspace
even in cases where this could be handled via netlink policy.

Convert a bunch of 'flag' attributes to use NLA_POLICY_MASK checks.

Signed-off-by: Florian Westphal <fw@strlen.de>