lightnvm: rename ->nr_pages to ->nr_sects

The struct rrpc->nr_pages can easily be interpreted as the number of
flash pages allocated to rrpc, while it is the nr_sects. Make sure that
this is reflected from the variable name.

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