vlan: introduce vid list with reference counting

This allows to keep track of vids needed to be in rx vlan filters of
devices even if they are used in bond/team etc.

vlan_info as well as vlan_group previously was, is allocated when first
vid is added and dealocated whan last vid is deleted.

vlan_group definition is moved to private header.

Signed-off-by: Jiri Pirko <jpirko@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>