Shutdown VLAN's before the actual/parent interface

Make sure the VLAN interfaces as well as its rules will be removed prior
to the actual/parent interface.
The VLAN configs are gone in vlan_post_stop() already so we have to use
vlan_pre_stop() instead.

Reported-by: Bernd Bednarz <bernd.bednarz@avira.com>
X-Gentoo-Bug: 380603
X-Gentoo-Bug-URL: https://bugs.gentoo.org/380603

net/vlan.sh

@@ -101,10 +101,12 @@ vlan_post_start()
 	return 0
 }
 
-vlan_post_stop()
+vlan_pre_stop()
 {
 	local vlan=
 
+	_exists || return 1
+
 	for vlan in $(_get_vlans); do
 		einfo "Removing VLAN ${vlan##*.} from ${IFACE}"
 		(