netfilter: xtables: slightly better error reporting

When extended status codes are available, such as ENOMEM on failed
allocations, or subsequent functions (e.g. nf_ct_get_l3proto), passing
them up to userspace seems like a good idea compared to just always
EINVAL.

Signed-off-by: Jan Engelhardt <jengelh@medozas.de>
diff --git a/net/netfilter/xt_conntrack.c b/net/netfilter/xt_conntrack.c
index 26e34aa..e0bcf8d 100644
--- a/net/netfilter/xt_conntrack.c
+++ b/net/netfilter/xt_conntrack.c
@@ -208,10 +208,13 @@
 
 static int conntrack_mt_check(const struct xt_mtchk_param *par)
 {
-	if (nf_ct_l3proto_try_module_get(par->family) < 0) {
+	int ret;
+
+	ret = nf_ct_l3proto_try_module_get(par->family);
+	if (ret < 0) {
 		pr_info("cannot load conntrack support for proto=%u\n",
 			par->family);
-		return -EINVAL;
+		return ret;
 	}
 	return 0;
 }