mac80211: deinitialize ibss-internals after emptiness check

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
f3209bea
("mac80211: fix IBSS teardown race")

Cc: stable@vger.kernel.org
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>