[VLAN]: translate IF_OPER_DORMANT to netif_dormant_on()

this patch adds support to the VLAN driver to translate IF_OPER_DORMANT of the
underlying device to netif_dormant_on(). Beside clean state forwarding, this
allows running independant userspace supplicants on both the real device and
the stacked VLAN. It depends on my RFC2863 patch.

Signed-off-by: Stefan Rompf <stefan@loplof.de>
Signed-off-by: David S. Miller <davem@davemloft.net>