s390/dasd: fix memory leak caused by dangling references to request_queue

After the call to del_gendisk, the gendisk still holds a reference to
its request_queue. We must not modify the gendisks queue pointer
before the put_disk call, or the gendisk_release function cannot
release the reference and the memory for the request_queue structure
is lost.

Signed-off-by: Stefan Weinhuber <wein@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>