bridge: Track inter-BSS usage

bridge: Track inter-BSS usage

Currently, struct hostapd_vlan is a per-BSS data structure which
also contains informations about whether to remove the bridge
or clear wlan / tagged-vlan interface from the bridge.

In a multi-interface multi-BSS setup, this can lead to the following
race condition:
 1. wlan0 creates VLAN A, sets DVLAN_CLEAN_BR and DVLAN_CLEAN_VLAN_PORT
 2. wlan1 creates VLAN A, does not set DVLAN_CLEAN_BR and
    DVLAN_CLEAN_VLAN_PORT as already there
 3. wlan0 removes VLAN A, removes tagged-interface from the bridge
    but not the bridge.
    Now wlan1 VLAN A is unusable due to the missing uplink.
 4. wlan1 removes VLAN A, does not cleanup

Solution:
This requires an inter-BSS inter-interface data structure to track the
bridge / bridge port usage within hostapd. This data structure could
also be used to track any other device-has-been-created-by-hostapd
information or when regarding interface freeing.

Signed-hostap: Michael Braun <michael-dev@fami-braun.de>