Fix netd mode on Android 6 with repeater

mobile/src/main/java/be/mygod/vpnhotspot/net/Routing.kt

@@ -115,15 +115,16 @@ class Routing(private val caller: Any, private val downstream: String) : IpNeigh
                             // by the time stopScript is called, table entry for upstream may already get removed
                             "ip rule del from all iif $downstream priority $priority")
                 }
-                when (masqueradeMode) {
-                    MasqueradeMode.None -> { }  // nothing to be done here
+                fun simpleMasquerade() {
                     // note: specifying -i wouldn't work for POSTROUTING
-                    MasqueradeMode.Simple -> {
                     iptablesAdd(if (upstream == null) "vpnhotspot_masquerade -s $hostSubnet -j MASQUERADE" else
                         "vpnhotspot_masquerade -s $hostSubnet -o $upstream -j MASQUERADE", "nat")
                 }
-                    MasqueradeMode.Netd -> {
-                        check(upstream != null)
+                when (masqueradeMode) {
+                    MasqueradeMode.None -> { }  // nothing to be done here
+                    MasqueradeMode.Simple -> simpleMasquerade()
+                    // fallback is only needed for repeater on API 23
+                    MasqueradeMode.Netd -> if (upstream == null) simpleMasquerade() else {
                         /**
                          * 0 means that there are no interface addresses coming after, which is unused anyway.
                          *