net: introduce pre-up netdev notifier

NETDEV_UP is called after the device is set UP, but sometimes
it is useful to be able to veto the device UP. Introduce a
new NETDEV_PRE_UP notifier that can be used for exactly this.
The first use case will be cfg80211 denying interfaces to be
set UP if the device is known to be rfkill'ed.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>