src: ensure chain policy evaluation when specified

src: ensure chain policy evaluation when specified

Set on CHAIN_F_BASECHAIN when policy is specified in chain, otherwise
chain priority is not evaluated.

Toggling this flag requires needs three adjustments to work though:

1) chain_evaluate() needs skip evaluation of hook name and priority if
   not specified to allow for updating the default chain policy, e.g.

	chain ip x y { policy accept; }

2) update netlink bytecode generation for chain to skip NFTA_CHAIN_HOOK
   so update path is exercised in the kernel.

3) error reporting needs to check if basechain priority and type is
   set on, otherwise skip further hints.

Fixes: acdfae9c3126 ("src: allow to specify the default policy for base chains")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>