mac80211: cooperate more with network namespaces

There are still two places in mac80211 that hardcode
the initial net namespace (init_net). One of them is
mandated by cfg80211 and will be removed by a separate
patch, the other one is used for finding the network
device of a pending packet via its ifindex.

Remove the latter use by keeping track of the device
pointer itself, via the vif pointer, and avoid it
going stale by dropping pending frames for a given
interface when the interface is removed.

To keep track of the vif pointer for the correct
interface, change the info->control.vif pointer's
internal use to always be the correct vif, and only
move it to the vif the driver expects (or NULL for
monitor interfaces and injected packets) right before
giving the packet to the driver.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>