mac80211: deinitialize ibss-internals after emptiness check

commit b78a4932f5fb11fadf41e69c606a33fa6787574c upstream.

The check whether the IBSS is active and can be removed should be
performed before deinitializing the fields used for the check/search.
Otherwise, the configured BSS will not be found and removed properly.

To make it more clear for the future, rename sdata->u.ibss to the
local pointer ifibss which is used within the checks.

This behaviour was introduced by
f3209bea110cade12e2b133da8b8499689cb0e2e
("mac80211: fix IBSS teardown race")

Cc: Ignacy Gawedzki <i@lri.fr>
Signed-off-by: Simon Wunderlich <siwu@hrz.tu-chemnitz.de>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>