mac80211: filter multicast data packets on AP / AP_VLAN

This patch adds filtering for multicast data packets on AP_VLAN
interfaces that have no authorized station connected and changes
filtering on AP interfaces to not count stations assigned to
AP_VLAN interfaces.

This saves airtime and avoids waking up other stations currently
authorized in this BSS. When using WPA, the packets dropped could
not be decrypted by any station.

The behaviour when there are no AP_VLAN interfaces is left unchanged.

When there are AP_VLAN interfaces, this patch
1. adds filtering multicast data packets sent on AP_VLAN interfaces
   that have no authorized station connected.
   No filtering happens on 4addr AP_VLAN interfaces.
2. makes filtering of multicast data packets sent on AP interfaces
   depend on the number of authorized stations in this bss not
   assigned to an AP_VLAN interface.

Therefore, a new num_mcast_sta counter is added for AP_VLAN interfaces.
The existing one for AP interfaces is altered to not track stations
assigned to an AP_VLAN interface.

The new counter is exposed in debugfs.

Signed-off-by: Michael Braun <michael-dev@fami-braun.de>
[reformat commit message a bit, unline ieee80211_vif_{inc,dec}_num_mcast]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>