Terminate the UBI background thread prior to restarting the system.

[Artem: amended comments a little]
Signed-off-by: NKevin Cernekee <kpc.mtd@gmail.com>
Tested-by: NArtem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: NArtem Bityutskiy <Artem.Bityutskiy@nokia.com>