netfilter: nft_synproxy: unregister hooks on init error path

Disable the IPv4 hooks if the IPv6 hooks fail to be registered.

Fixes: ad49d86e07a4 ("netfilter: nf_tables: Add synproxy support")
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

net/netfilter/nft_synproxy.c

···
 		if (err)
 			goto nf_ct_failure;
 		err = nf_synproxy_ipv6_init(snet, ctx->net);
-		if (err)
+		if (err) {
+			nf_synproxy_ipv4_fini(snet, ctx->net);
 			goto nf_ct_failure;
+		}
 		break;
 	}