mac80211: fix SMPS debugfs locking

The locking with SMPS requests means that the
debugs file should lock the mgd mutex, not the
iflist mutex. Calls to __ieee80211_request_smps()
need to hold that mutex, so add an assertion.

This has always been wrong, but for some reason
never been noticed, probably because the locking
error only happens while unassociated.

Cc: stable@kernel.org [2.6.34+]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>

net/mac80211/cfg.c

@@ -1504,6 +1504,8 @@ int __ieee80211_request_smps(struct ieee80211_sub_if_data *sdata,
 	enum ieee80211_smps_mode old_req;
 	int err;
 
+	lockdep_assert_held(&sdata->u.mgd.mtx);
+
 	old_req = sdata->u.mgd.req_smps;
 	sdata->u.mgd.req_smps = smps_mode;