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>