lightnvm: pblk: calculate line pad distance in helper

If a line is padded, calculate the pad distance directly on the helper
being used for this purpose.

Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>