core: don't bounce disable_ipv6 when assuming connections (rh #1170530)

Don't call set_nm_ipv6ll(self, TRUE) on any assumed connection since it
would bounce disable_ipv6, which would break IPv6 connectivity.
That is critical, for example, for installations via NFS.

Fixes: d37b7bed30

https://bugzilla.redhat.com/show_bug.cgi?id=1170530
Related: https://bugzilla.redhat.com/show_bug.cgi?id=1052157

src/devices/nm-device.c

@@ -4390,7 +4390,7 @@ act_stage3_ip6_config_start (NMDevice *self,
 	 * IPv6LL if this is not an assumed connection, since assumed connections
 	 * will already have IPv6 set up.
 	 */
-	if (!nm_device_uses_generated_assumed_connection (self))
+	if (!nm_device_uses_assumed_connection (self))
 		set_nm_ipv6ll (self, TRUE);
 
 	/* Re-enable IPv6 on the interface */