mac80211: Fix crash due to un-canceled work-items

Some mlme work structs are not cancelled on disassociation
nor interface deletion, which leads to them running after
the memory has been freed

There is not a clean way to cancel these in the disassociation
logic because they must be canceled outside of the ifmgd->mtx
lock, so just cancel them in mgd_stop logic that tears down
the station.

This fixes the crashes we see in 3.7.9+.  The crash stack
trace itself isn't so helpful, but this warning gives
more useful info:

WARNING: at /home/greearb/git/linux-3.7.dev.y/lib/debugobjects.c:261 debug_print_object+0x7c/0x8d()
ODEBUG: free active (active state 0) object type: work_struct hint: ieee80211_sta_monitor_work+0x0/0x14 [mac80211]
Modules linked in: [...]
Pid: 14743, comm: iw Tainted: G         C O 3.7.9+ #11
Call Trace:
 [<ffffffff81087ef8>] warn_slowpath_common+0x80/0x98
 [<ffffffff81087fa4>] warn_slowpath_fmt+0x41/0x43
 [<ffffffff812a2608>] debug_print_object+0x7c/0x8d
 [<ffffffff812a2bca>] debug_check_no_obj_freed+0x95/0x1c3
 [<ffffffff8114cc69>] slab_free_hook+0x70/0x79
 [<ffffffff8114ea3e>] kfree+0x62/0xb7
 [<ffffffff8149f465>] netdev_release+0x39/0x3e
 [<ffffffff8136ad67>] device_release+0x52/0x8a
 [<ffffffff812937db>] kobject_release+0x121/0x158
 [<ffffffff81293612>] kobject_put+0x4c/0x50
 [<ffffffff8148f0d7>] netdev_run_todo+0x25c/0x27e

Cc: stable@vger.kernel.org
Signed-off-by: Ben Greear <greearb@candelatech.com>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/mlme.c

@@ -4319,6 +4319,17 @@ void ieee80211_mgd_stop(struct ieee80211_sub_if_data *sdata)
 {
 	struct ieee80211_if_managed *ifmgd = &sdata->u.mgd;
 
+	/*
+	 * Make sure some work items will not run after this,
+	 * they will not do anything but might not have been
+	 * cancelled when disconnecting.
+	 */
+	cancel_work_sync(&ifmgd->monitor_work);
+	cancel_work_sync(&ifmgd->beacon_connection_loss_work);
+	cancel_work_sync(&ifmgd->request_smps_work);
+	cancel_work_sync(&ifmgd->csa_connection_drop_work);
+	cancel_work_sync(&ifmgd->chswitch_work);
+
 	mutex_lock(&ifmgd->mtx);
 	if (ifmgd->assoc_data)
 		ieee80211_destroy_assoc_data(sdata, false);