block: discard bdi_unregister() in favour of bdi_destroy()

bdi_unregister() now contains very little functionality.

It contains a "WARN_ON" if bdi->dev is NULL.  This warning is of no
real consequence as bdi->dev isn't needed by anything else in the function,
and it triggers if
   blk_cleanup_queue() -> bdi_destroy()
is called before bdi_unregister, which happens since
  Commit: 6cd18e71 ("block: destroy bdi before blockdev is unregistered.")

So this isn't wanted.

It also calls bdi_set_min_ratio().  This needs to be called after
writes through the bdi have all been flushed, and before the bdi is destroyed.
Calling it early is better than calling it late as it frees up a global
resource.

Calling it immediately after bdi_wb_shutdown() in bdi_destroy()
perfectly fits these requirements.

So bdi_unregister() can be discarded with the important content moved to
bdi_destroy(), as can the
  writeback_bdi_unregister
event which is already not used.

Reported-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org (v4.0)
Fixes: c4db59d3 ("fs: don't reassign dirty inodes to default_backing_dev_info")
Fixes: 6cd18e71 ("block: destroy bdi before blockdev is unregistered.")
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Tested-by: Nicholas Moulin <nicholas.w.moulin@linux.intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@fb.com>