mac80211: optimise mixed AP/VLAN station removal

Teach sta_info_flush() to optionally also remove stations
from all VLANs associated with an AP interface to optimise
the station removal (in particular, synchronize_net().)

To not have to add the vlans argument throughout, do some
refactoring.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>

diff --cc net/mac80211/cfg.c
Simple merge

diff --cc net/mac80211/sta_info.c
Simple merge

diff --cc net/mac80211/sta_info.h
Simple merge