mac80211: track assigned vifs in chanctx

This can be useful. Provides a more straghtforward
way to iterate over interfaces bound to a given
chanctx and allows tracking chanctx usage
explicitly.

The structure is protected by local->chanctx_mtx.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

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

diff --cc net/mac80211/ieee80211_i.h
Simple merge

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