bdi: Shutdown writeback on all cgwbs in cgwb_bdi_destroy()

Currently we waited for all cgwbs to get freed in cgwb_bdi_destroy()
which also means that writeback has been shutdown on them. Since this
wait is going away, directly shutdown writeback on cgwbs from
cgwb_bdi_destroy() to avoid live writeback structures after
bdi_unregister() has finished. To make that safe with concurrent
shutdown from cgwb_release_workfn(), we also have to make sure
wb_shutdown() returns only after the bdi_writeback structure is really
shutdown.

Change-Id: I62d267ca6dc7919ae16fc3226f857238ab42a8f0
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Jan Kara <jack@suse.cz>
Signed-off-by: Jens Axboe <axboe@fb.com>
Git-commit: 5318ce7d46866e1dbc20ab9349b93753edba0b3e
Git-repo: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux-stable.git


Signed-off-by: Ritesh Harjani <riteshh@codeaurora.org>