scsi: scsi_debug: Make sdebug_build_parts() respect virtual_gb

If virtual_gb is passed while using num_parts, when creating the
partitions, virtual_gb is not respected.  Set num_sectors using
get_sdebug_capacity() to pull virtual_gb if set.

Link: https://lore.kernel.org/r/20200902211434.9979-3-jpittman@redhat.com


Acked-by: Douglas Gilbert <dgilbert@interlog.com>
Signed-off-by: John Pittman <jpittman@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>