writeback: expose the bdi_wq workqueue

There are cases where userland wants to tweak the priority and
affinity of writeback flushers.  Expose bdi_wq to userland by setting
WQ_SYSFS.  It appears under /sys/bus/workqueue/devices/writeback/ and
allows adjusting maximum concurrency level, cpumask and nice level.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Fengguang Wu <fengguang.wu@intel.com>
Cc: Jeff Moyer <jmoyer@redhat.com>
Cc: Kay Sievers <kay.sievers@vrfy.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

mm/backing-dev.c

@@ -259,7 +259,7 @@ static int __init default_bdi_init(void)
 	int err;
 
 	bdi_wq = alloc_workqueue("writeback", WQ_MEM_RECLAIM | WQ_FREEZABLE |
-					      WQ_UNBOUND, 0);
+					      WQ_UNBOUND | WQ_SYSFS, 0);
 	if (!bdi_wq)
 		return -ENOMEM;