zebra: clear ZEBRA_IFC_CONFIGURED on "no ipv6 addr"

To match the semantics of IPv4, the ZEBRA_IFC_CONFIGURED flag
should be cleared when an IPv6 connected is uninstalled via
vty.

Signed-off-by: Christian Franke <chris@opensourcerouting.org>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
diff --git a/zebra/interface.c b/zebra/interface.c
index baa7ab6..cd78ebb 100644
--- a/zebra/interface.c
+++ b/zebra/interface.c
@@ -1499,6 +1499,8 @@
   if (! CHECK_FLAG (ifc->conf, ZEBRA_IFC_CONFIGURED))
     return CMD_WARNING;
 
+  UNSET_FLAG (ifc->conf, ZEBRA_IFC_CONFIGURED);
+
   /* This is not real address or interface is not active. */
   if (! CHECK_FLAG (ifc->conf, ZEBRA_IFC_REAL)
       || ! CHECK_FLAG (ifp->status, ZEBRA_INTERFACE_ACTIVE))