s390/dasd: Use ALIGN_DOWN macro

There is now an ALIGN_DOWN macro available. Let's rather use kernel
provided macros that do the things we want.

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>