bcache: Shutdown fix

commit 5caa52af upstream.

Stopping a cache set is supposed to make it stop attached backing
devices, but somewhere along the way that code got lost. Fixing this
mainly has the effect of fixing our reboot notifier.

Signed-off-by: Kent Overstreet <kmo@daterainc.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>