netfilter: flowtable: skip offload setup if disabled

nftables test case
tests/shell/testcases/flowtable/0001flowtable_0

results in a crash. After the refactor, if we leave early via
nf_flowtable_hw_offload(), then "struct flow_block_offload" is left
in an uninitialized state, but later users assume its initialised.

Fixes: a7965d58 ("netfilter: flowtable: add nf_flow_table_offload_cmd()")
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nf_flow_table_offload.c

@@ -847,9 +847,6 @@ static int nf_flow_table_offload_cmd(struct flow_block_offload *bo,
 {
 	int err;
 
-	if (!nf_flowtable_hw_offload(flowtable))
-		return 0;
-
 	if (!dev->netdev_ops->ndo_setup_tc)
 		return -EOPNOTSUPP;
 
@@ -876,6 +873,9 @@ int nf_flow_table_offload_setup(struct nf_flowtable *flowtable,
 	struct flow_block_offload bo;
 	int err;
 
+	if (!nf_flowtable_hw_offload(flowtable))
+		return 0;
+
 	err = nf_flow_table_offload_cmd(&bo, flowtable, dev, cmd, &extack);
 	if (err < 0)
 		return err;