s390/dasd: Add discard support for ESE volumes

ESE (Extent Space Efficient) volumes are thin-provisioned and therefore
space is only occupied with real data. In order to make previously used
space available for re-allocation again, discard support is enabled for
ESE volumes allowing the DASD driver to release said space.

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>