scsi: arm: Move the SCSI pointer to private command data

Set .cmd_size in the SCSI host template instead of using the SCSI pointer
from struct scsi_cmnd. This patch prepares for removal of the SCSI pointer
from struct scsi_cmnd. The ARM SCSI drivers have been identified as follows:
$ git grep -l '#include.*arm_scsi.h'
drivers/scsi/arm/acornscsi.c
drivers/scsi/arm/cumana_2.c
drivers/scsi/arm/eesox.c
drivers/scsi/arm/fas216.c
drivers/scsi/arm/powertec.c

Link: https://lore.kernel.org/r/20220218195117.25689-11-bvanassche@acm.org
Cc: Russell King <linux@armlinux.org.uk>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Reviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>

scsi: arm: Rename arm/scsi.h into arm/arm_scsi.h

The new name makes the purpose of this header file more clear and also
makes it easier to find this header file with grep.

Link: https://lore.kernel.org/r/20220218195117.25689-10-bvanassche@acm.org
Cc: Russell King <linux@armlinux.org.uk>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Himanshu Madhani <himanshu.madhani@oracle.com>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>