lightnvm: pblk: rename ppaf* to addrf*

In preparation for 2.0 support in pblk, rename variables referring to
the address format to addrf and reserve ppaf for the 1.2 path.

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