s390 dasd driver fixes:
 - Take request queue lock in dasd_end_request.
 - Make it work with CONFIG_DEVFS_FS=y.
 - Properly wait for the root device.
 - Cope with requests killed due to failed channel path.
 - Improve reference counting.
 - Remove devno from struct dasd_device.
 - Remove unnecessary bdget/bdput calls.