vlan: T3018: vif/vif-s vlan id can not be re-used

In the past it was possible to configure a vif-s interface and a vif interface
both with the same VLAN ID. VyOS 1.2 reported a Kernel error:

RTNETLINK answers: File exists
Error creating VLAN device eth1.100

so this should not be possible at all in VyOS 1.3

(cherry picked from commit ccd516b4d10c518ea445928c01d6c7dc2770777b)