lightnvm: move ppa transformations to core

Continuing the effort of moving 1.2 and 2.0 specific code to core, move
64_to_32 and 32_to_64 ppa helpers from pblk to core.

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