mtd: blktrans: fix a race vs kthread_stop

There is small race window that could make kthread_stop hang forever.
I found that while hacking the IR subsystem.

Signed-off-by: Maxim Levitsky <maximlevisky@gmail.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>