UBI: Don't exit from ubi_thread until kthread_should_stop() is true

If ubi_thread() exits but kthread_should_stop() is not true
then kthread_stop() will never return and cleanup thread
will forever stay in "D" state.

Signed-off-by: Vitaliy Gusev <vgusev@openvz.org>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>