netfilter: nf_tables_offload: return EOPNOTSUPP if rule specifies no actions

If the rule only specifies the matching side, return EOPNOTSUPP.
Otherwise, the front-end relies on the drivers to reject this rule.

Fixes: c9626a2c

 ("netfilter: nf_tables: add hardware offload support")
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>