By putting the fs devices/drivers on a bus on its own we can hook
into the bus remove function to cleanup the fs device. This way
we can unmount a device by simply unregistering the device which
is useful for for example USB mass storage devices. These can now
unregister the assoiated filesystems by unregistering their child
devices.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>