Replace DATA_PAGES_PER_BLK and DATA_BLOCK_SIZE with new struct elements
tcmu_dev->data_pages_per_blk and tcmu_dev->data_blk_size.  These new
variables are still loaded with constant definition DATA_PAGES_PER_BLK_DEF
(= 1) and DATA_PAGES_PER_BLK_DEF * PAGE_SIZE.

There is no way yet to set the values via configfs.

Link: https://lore.kernel.org/r/20210324195758.2021-6-bostroesser@gmail.comSigned-off-by: Bodo Stroesser <bostroesser@gmail.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>