scsi: target: core: Prevent underflow for service actions

TCM buffer length doesn't necessarily equal 8 + ADDITIONAL LENGTH which
might be considered an underflow in case of Data-In size being greater than
8 + ADDITIONAL LENGTH. So truncate buffer length to prevent underflow.

Link: https://lore.kernel.org/r/20210209072202.41154-3-a.miloserdov@yadro.com
Reviewed-by: Roman Bolshakov <r.bolshakov@yadro.com>
Reviewed-by: Bodo Stroesser <bostroesser@gmail.com>
Signed-off-by: Aleksandr Miloserdov <a.miloserdov@yadro.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>