mac80211: fix cfg80211 interaction on auth/assoc request

If authentication (or association with FT) is requested by
userspace, mac80211 currently doesn't tell cfg80211 that it
disconnected from the AP. That leaves inconsistent state:
cfg80211 thinks it's connected while mac80211 thinks it's
not. Typically this won't last long, as soon as mac80211
reports the new association to cfg80211 the old one goes
away. If, however, the new authentication or association
doesn't succeed, then cfg80211 will forever think the old
one still exists and will refuse attempts to authenticate
or associate with the AP it thinks it's connected to.

Anders reported that this leads to it taking a very long
time to reconnect to a network, or never even succeeding.
I tested this with an AP hacked to never respond to auth
frames, and one that works, and with just those two the
system never recovers because one won't work and cfg80211
thinks it's connected to the other so refuses connections
to it.

To fix this, simply make mac80211 tell cfg80211 when it is
no longer connected to the old AP, while authenticating or
associating to a new one.

Cc: stable@vger.kernel.org
Reported-by: Anders Kaseorg <andersk@mit.edu>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>