netfilter: nf_tables: Utilize NLA_POLICY_NESTED_ARRAY

Mark attributes which are supposed to be arrays of nested attributes
with known content as such. Originally suggested for
NFTA_RULE_EXPRESSIONS only, but does apply to others as well.

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