mac80211: update mesh beacon on workqueue

Instead of updating the mesh beacon immediately when
requested (which would require the sdata_lock()), defer it
to the mac80211 workqueue.

Fixes yet another deadlock on calling sta_info_flush()
with the sdata_lock() held from ieee80211_stop_mesh(). We
could just drop the sdata_lock() around the
mesh_sta_cleanup() call, but this path is also taken from
several non-locked error paths.

Signed-off-by: Thomas Pedersen <thomas@cozybit.com>
[fix comment position]
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

net/mac80211/mesh.h

@@ -58,6 +58,7 @@ enum mesh_path_flags {
  * @MESH_WORK_ROOT: the mesh root station needs to send a frame
  * @MESH_WORK_DRIFT_ADJUST: time to compensate for clock drift relative to other
  * mesh nodes
+ * @MESH_WORK_MBSS_CHANGED: rebuild beacon and notify driver of BSS changes
  */
 enum mesh_deferred_task_flags {
 	MESH_WORK_HOUSEKEEPING,
@@ -65,6 +66,7 @@ enum mesh_deferred_task_flags {
 	MESH_WORK_GROW_MPP_TABLE,
 	MESH_WORK_ROOT,
 	MESH_WORK_DRIFT_ADJUST,
+	MESH_WORK_MBSS_CHANGED,
 };
 
 /**