lightnvm: support multiple ppas in nvm_erase_ppa

Sometimes a user want to erase multiple PPAs at the same time. Extend
nvm_erase_ppa to take multiple ppas and number of ppas to be erased.

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