null_blk: pass transfer size to null_handle_rq()

As preparation to support partial data transfer, add a new argument to
null_handle_rq() to pass the number of sectors to transfer. While at it,
rename the function from null_handle_rq to null_handle_data_transfer.
This commit does not change the behavior.

Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Shin'ichiro Kawasaki <shinichiro.kawasaki@wdc.com>
Link: https://lore.kernel.org/r/20250226100613.1622564-5-shinichiro.kawasaki@wdc.com


Signed-off-by: Jens Axboe <axboe@kernel.dk>