lightnvm: refactor set_bb_tbl for accepting ppa list

The set_bb_tbl takes struct nvm_rq and only uses its ppa_list and
nr_pages internally. Instead, make these two variables explicit.
This allows a user to call it without initializing a struct nvm_rq
first.

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