s390/dasd: simplify locking in dasd_times_out

Provide __dasd_cancel_req that is called with the ccw device lock
held to simplify the locking in dasd_times_out. Also this removes
the following sparse warning:
context imbalance in 'dasd_times_out' - different lock contexts for basic block

Note: with this change dasd_schedule_device_bh is now called (via
dasd_cancel_req) with the ccw device lock held. But is is already
the case for other codepaths.

Signed-off-by: Sebastian Ott <sebott@linux.ibm.com>
Reviewed-by: Stefan Haberland <sth@linux.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>