pc-bios/s390-ccw: fix for fragmented SCSI bootmap

pc-bios/s390-ccw: fix for fragmented SCSI bootmap

We need to interpret the last entry of the bootmap with zero
block count as "continuation pointer".
The "last entry" is being detected by pre-filling of the scratch
space with known values and respective look-ahead.

Signed-off-by: Eugene (jno) Dvurechenski <jno@linux.vnet.ibm.com>
Signed-off-by: Jens Freimann <jfrei@linux.vnet.ibm.com>
Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Reviewed-by: Cornelia Huck <cornelia.huck@de.ibm.com>
Reviewed-by: Alexander Graf <agraf@suse.de>
Signed-off-by: Cornelia Huck <cornelia.huck@de.ibm.com>