network: only check for IPv6 RA routes when the network has an IPv6 address

commit 00d28a78 added a check to see if there were any IPv6 routes
added by RA (Router Advertisement) via an interface that had accept_ra
set to something other than "2". The check was being done
unconditionally, but it's only relevant if IPv6 forwarding is going to
be turned on, and that will only happen if the network has an IPv6
address.

src/network/bridge_driver.c

@@ -2391,10 +2391,9 @@ networkStartNetworkVirtual(virNetworkDriverStatePtr driver,
 
     /* If forward.type != NONE, turn on global IP forwarding */
     if (network->def->forward.type != VIR_NETWORK_FORWARD_NONE) {
-        if (!virNetDevIPCheckIPv6Forwarding())
+        if (v6present && !virNetDevIPCheckIPv6Forwarding())
             goto err3; /* Precise error message already provided */
 
-
         if (networkEnableIPForwarding(v4present, v6present) < 0) {
             virReportSystemError(errno, "%s",
                                  _("failed to enable IP forwarding"));