[SCSI] scsi_debug: prohibit scsi_debug_unmap_granularity == scsi_debug_unmap_alignment

scsi_debug prohibits setting scsi_debug_unmap_alignment to be greater
than scsi_debug_unmap_granularity.  But setting them to be the same value
is not prohibited.  In this case, the only difference with
scsi_debug_unmap_alignment == 0 is the logical blocks from 0 to
scsi_debug_unmap_alignment - 1 cannot be unmapped.  But the difference is
not properly handled in the current code.

So this prohibits such unusual setting.

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>