There is dasd_sleep_on() and dasd_sleep_on_interruptible() to start CCW
requests uninterruptible and interruptible. However, there is only
dasd_sleep_on_queue() to start requests from CCW queues uninterruptible.

Add dasd_sleep_on_queue_interruptible() to provide a way to start
requests from CCW queues interruptible. _dasd_sleep_on_queue() already
provides this functionality.
Signed-off-by: Jan Höppner <hoeppner@linux.ibm.com>
Reviewed-by: Stefan Haberland <sth@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>