bdi: make backing_dev_info->wb.dwork canceling stricter

Canceling of bdi->wb.dwork is currently a bit mushy.
bdi_wb_shutdown() performs cancel_delayed_work_sync() at the end after
shutting down and flushing the delayed_work and bdi_destroy() tries
yet again after bdi_unregister().

bdi->wb.dwork is queued only after checking BDI_registered while
holding bdi->wb_lock and bdi_wb_shutdown() clears the flag while
holding the same lock and then flushes the delayed_work.  There's no
way the delayed_work can be queued again after that.

Replace the two unnecessary cancel_delayed_work_sync() invocations
with WARNs on pending.  This simplifies and clarifies the code a bit
and will help future changes in further isolating bdi_writeback
handling.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jens Axboe <axboe@fb.com>