MFC r215726:

MFC r215726:

  Allow for vlan(4) ifnets to have overlapping unit numbers if they are
  created in separated vnets.  As a side-effect of having a separated
  if_cloner instance for each vnet, all vlan ifnets created in a vnet
  will be automatically destroyed when vnet teardown is initiated.

  Disallow SIOCSETVLAN and SIOCGETVLAN ioctls on vlan ifnets which are
  associated with physical ifnets residing in parent vnets.

  This is an interim vlan-specific solution which will be superseded by a
  more generic if_cloner V_irtualization change from p4.  For nooptions
  VIMAGE builds, this should be a no-op change.

  Discussed with:       bz

git-svn-id: svn://svn.freebsd.org/base/stable/8@215871 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f