net: Report dev->promiscuity in netlink reports.

The standard ways of probing a device's promiscuity
(ifi_flags, for instance) does not report the actual
state of the device.  This patch adds dev->promiscuity
to the netlink netdevice report so that users can know
for certain if the device is acting PROMISC or not.

Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: David S. Miller <davem@davemloft.net>