ipv4: fix error return code in rtm_to_fib_config()

Fix to return a negative error code from the error handling
case instead of 0, as done elsewhere in this function.

Fixes: d1566268 ("ipv4: Allow ipv6 gateway with ipv4 routes")
Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Zhang Changzhong <zhangchangzhong@huawei.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/1607071695-33740-1-git-send-email-zhangchangzhong@huawei.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/fib_frontend.c

@@ -825,7 +825,7 @@ static int rtm_to_fib_config(struct net *net, struct sk_buff *skb,
 	if (has_gw && has_via) {
 		NL_SET_ERR_MSG(extack,
 			       "Nexthop configuration can not contain both GATEWAY and VIA");
-		goto errout;
+		return -EINVAL;
 	}
 
 	return 0;