netfilter: nf_tables: allow to change chain policy without hook if it exists

[ Upstream commit d6b6cb1d3e6f78d55c2d4043d77d0d8def3f3b99 ]

If there's an existing base chain, we have to allow to change the
default policy without indicating the hook information.

However, if the chain doesn't exists, we have to enforce the presence of
the hook attribute.

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>