mac80211 calls synchronize_rcu() on sta deletion,
which increase the roaming time significantly.
Convert it into a call_rcu() mechanism, in order
to avoid blocking. Since some of the cleanup
functions might sleep, schedule from the call_rcu
callback a new work that will do the actual cleanup.
In order to make sure the cleanup occurs before
the interface went down, flush local->workqueue
on ieee80211_do_stop().