lightnvm: rename dma helper functions

Until now, the dma pool have been exclusively used to allocate the ppa
list being sent to the device. In pblk (upcoming), we use these pools to
allocate metadata too. Thus, we generalize the names of some variables
on the dma helper functions to make the code more readable.

Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>