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: c9626a2cbdb2 ("netfilter: nf_tables: add hardware offload support")
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/netfilter/nf_tables_offload.c

···
 		expr = nft_expr_next(expr);
 	}
 
+	if (num_actions == 0)
+		return ERR_PTR(-EOPNOTSUPP);
+
 	flow = nft_flow_rule_alloc(num_actions);
 	if (!flow)
 		return ERR_PTR(-ENOMEM);