ipv4: Handle RTA_GATEWAY set to 0

Govindarajulu reported a regression with Network Manager which sends an
RTA_GATEWAY attribute with the address set to 0. Fixup the handling of
RTA_GATEWAY to only set fc_gw_family if the gateway address is actually
set.

Fixes: f35b794b ("ipv4: Prepare fib_config for IPv6 gateway")
Reported-by: Govindarajulu Varadarajan <govind.varadar@gmail.com>
Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/fib_frontend.c

@@ -755,8 +755,9 @@ static int rtm_to_fib_config(struct net *net, struct sk_buff *skb,
 			break;
 		case RTA_GATEWAY:
 			has_gw = true;
-			cfg->fc_gw_family = AF_INET;
 			cfg->fc_gw4 = nla_get_be32(attr);
+			if (cfg->fc_gw4)
+				cfg->fc_gw_family = AF_INET;
 			break;
 		case RTA_VIA:
 			has_via = true;

net/ipv4/fib_semantics.c

@@ -616,8 +616,9 @@ static int fib_get_nhs(struct fib_info *fi, struct rtnexthop *rtnh,
 				return -EINVAL;
 			}
 			if (nla) {
-				fib_cfg.fc_gw_family = AF_INET;
 				fib_cfg.fc_gw4 = nla_get_in_addr(nla);
+				if (fib_cfg.fc_gw4)
+					fib_cfg.fc_gw_family = AF_INET;
 			} else if (nlav) {
 				ret = fib_gw_from_via(&fib_cfg, nlav, extack);
 				if (ret)