[SCSI] scsi_debug: clear correct memory region when LBPRZ is enabled

The function unmap_region() clears memory region specified as the logical
block address and the number of logical blocks in ramdisk storage
(fake_storep) if lbpu and lbprz module parameters are enabled.

In the while loop of unmap_region(), it advances optimal unmap granularity
in logical blocks.  But it only clears one logical block at LBA 'block' per
loop iteration.  And furthermore, the 'block' is not pointing to a logical
block address which should be cleared, it is a index of probisioning map
(map_storep).

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Acked-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>