scsi: target: tcmu: Make data_pages_per_blk changeable via configfs

Make data_pages_per_blk changeable similar to the way it is done for
max_data_area_mb. One can change the value by typing:

  echo "data_pages_per_blk=N" >control

The value is printed when doing:

  cat info

In addition, a new readonly attribute 'data_pages_per_blk' returns the
value on read.

Link: https://lore.kernel.org/r/20210324195758.2021-7-bostroesser@gmail.com


Signed-off-by: Bodo Stroesser <bostroesser@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>