s390/dasd: only use preallocated requests

Change the remaining users of dasd_kmalloc_request to use
preallocated memory and remove this function.

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>