mac80211: properly flush delayed scan work on interface removal

mac80211: properly flush delayed scan work on interface removal

commit 46238845bd609a5c0fbe076e1b82b4c5b33360b2 upstream.

When an interface is deleted, an ongoing hardware scan is canceled and
the driver must abort the scan, at the very least reporting completion
while the interface is removed.

However, if it scheduled the work that might only run after everything
is said and done, which leads to cfg80211 warning that the scan isn't
reported as finished yet; this is no fault of the driver, it already
did, but mac80211 hasn't processed it.

To fix this situation, flush the delayed work when the interface being
removed is the one that was executing the scan.

Reported-by: Sujith Manoharan <sujith@msujith.org>
Tested-by: Sujith Manoharan <sujith@msujith.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
[bwh: Backported to 3.2:
 - No rcu_access_pointer() needed
 - Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>