net/interfaces: always return an IPv4 LikelyHomeRouterIP

We weren't filtering out IPv6 addresses from this function, so we could
be returning an IPv4 gateway IP and an IPv6 self IP. Per the function
comments, only return IPv4 addresses for the self IP.

Signed-off-by: Andrew Dunham <andrew@du.nham.ca>
Change-Id: If19a4aadc343fbd4383fc5290befa0eff006799e

net/interfaces/interfaces.go

@@ -613,8 +613,18 @@ func LikelyHomeRouterIP() (gateway, myIP netip.Addr, ok bool) {
 		return
 	}
 	ForeachInterfaceAddress(func(i Interface, pfx netip.Prefix) {
+		if !i.IsUp() {
+			// Skip interfaces that aren't up.
+			return
+		} else if myIP.IsValid() {
+			// We already have a valid self IP; skip this one.
+			return
+		}
+
 		ip := pfx.Addr()
-		if !i.IsUp() || !ip.IsValid() || myIP.IsValid() {
+		if !ip.IsValid() || !ip.Is4() {
+			// Skip IPs that aren't valid or aren't IPv4, since we
+			// always return an IPv4 address.
 			return
 		}
 		if gateway.IsPrivate() && ip.IsPrivate() {