bdi: explain the dirty list transferring in bdi_destroy()

bdi_destroy() has code to transfer the remaining dirty inodes to the
default_backing_dev_info; however, given the shutdown sequence, it
isn't clear how such condition would happen.  Also, it isn't a full
solution as the transferred inodes stlil point to the bdi which is
being destroyed.  Operations on those inodes can end up accessing
already released fields such as the percpu stat fields.

Digging through the history, it seems that the code was added as a
quick workaround for a bug report without fully root-causing the
issue.  We probably want to remove the code in time but for now let's
add a comment noting that it is a quick workaround.

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