wifi: cfg80211: fix cqm_config access race

Max Schulze reports crashes with brcmfmac. The reason seems
to be a race between userspace removing the CQM config and
the driver calling cfg80211_cqm_rssi_notify(), where if the
data is freed while cfg80211_cqm_rssi_notify() runs it will
crash since it assumes wdev->cqm_config is set. This can't
be fixed with a simple non-NULL check since there's nothing
we can do for locking easily, so use RCU instead to protect
the pointer, but that requires pulling the updates out into
an asynchronous worker so they can sleep and call back into
the driver.

Since we need to change the free anyway, also change it to
go back to the old settings if changing the settings fails.

Reported-and-tested-by: Max Schulze <max.schulze@online.de>
Closes: https://lore.kernel.org/r/ac96309a-8d8d-4435-36e6-6d152eb31876@online.de
Fixes: 4a4b8169

 ("cfg80211: Accept multiple RSSI thresholds for CQM")
Signed-off-by: Johannes Berg <johannes.berg@intel.com>