writeback: eliminate work item allocation in bd_start_writeback()

Handle start-all writeback like we do periodic or kupdate
style writeback - by marking the bdi_writeback as needing a full
flush, and simply waking the thread. This eliminates the need to
allocate and queue a specific work item just for this purpose.

After this change, we truly only ever have one of them running at
any point in time. We mark the need to start all flushes, and the
writeback thread will clear it once it has processed the request.

Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

include/linux/writeback.h

@@ -41,28 +41,6 @@ enum writeback_sync_modes {
 	WB_SYNC_ALL,	/* Wait on every mapping */
 };
 
-/*
- * why some writeback work was initiated
- */
-enum wb_reason {
-	WB_REASON_BACKGROUND,
-	WB_REASON_VMSCAN,
-	WB_REASON_SYNC,
-	WB_REASON_PERIODIC,
-	WB_REASON_LAPTOP_TIMER,
-	WB_REASON_FREE_MORE_MEM,
-	WB_REASON_FS_FREE_SPACE,
-	/*
-	 * There is no bdi forker thread any more and works are done
-	 * by emergency worker, however, this is TPs userland visible
-	 * and we'll be exposing exactly the same information,
-	 * so it has a mismatch name.
-	 */
-	WB_REASON_FORKER_THREAD,
-
-	WB_REASON_MAX,
-};
-
 /*
  * A control structure which tells the writeback code what to do.  These are
  * always on the stack, and hence need no locking.  They are always initialised