netfilter: nf_flow_table: fix netdev refcnt leak

[ Upstream commit 26a302afbe328ecb7507cae2035d938e6635131b ]

flow_offload_alloc() calls nf_route() to get a dst_entry. Internally,
nf_route() calls ip_route_output_key() that allocates a dst_entry and
holds it. So, a dst_entry should be released by dst_release() if
nf_route() is successful.

Otherwise, netns exit routine cannot be finished and the following
message is printed:

[  257.490952] unregister_netdevice: waiting for lo to become free. Usage count = 1

Fixes: ac2a6666 ("netfilter: add generic flow table infrastructure")
Signed-off-by: NTaehee Yoo <ap420073@gmail.com>
Signed-off-by: NPablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: NSasha Levin <sashal@kernel.org>

net/netfilter/nft_flow_offload.c

@@ -113,6 +113,7 @@ static void nft_flow_offload_eval(const struct nft_expr *expr,
 	if (ret < 0)
 		goto err_flow_add;
 
+	dst_release(route.tuple[!dir].dst);
 	return;
 
 err_flow_add: