Keep track of the ifih corresponding to a vlan instance to ease its

Keep track of the ifih corresponding to a vlan instance to ease its
removal.

As soon as carp(4) will be converted to the new if_input() API it
will be possible to add multiple vlan(4) and carp(4) pseudo-ifps on
top of the same parent interface.  When such thing happens we can no
longer assume that the first pseudo-ifp to be destroyed will be the
last configured.

ok dlg@