Fix subinterface vlan creation.

D26436 introduced support for stacked vlans that changed the way vlans
are configured. In particular, this change broke setups that have
same-number vlans as subinterfaces.

Vlan support was initially created assuming "vlanX" semantics. In this paradigm,
automatic number assignment supported by cloning (ifconfig vlan create) was a
natural fit.
When "ifaceX.Y" support was added, allowing to have the same vlan number on
multiple devices, cloning code became more complex, as the is no
unified "vlan" namespace anymore. Such interfaces got the first spare
index from "vlan" cloner. This, in turn, led to the following problem:
ifconfig ix0.333 create -> index 1
ifconfig ix0.444 create -> index 2
ifconfig vlan2 create -> allocation failure

This change fixes such allocations by using cloning indexes only for
"vlanX" interfaces.

Reviewed by: hselasky
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D27505