lightnvm: calculate rrpc total blocks and sectors up front

Calculate rrpc total blocks and sectors up front, make sense
to use them. For example, we use rrpc->nr_sects to calculate rrpc
area size, but it makes no sense if we don't initialize it up front,
since it would be zero until we finish rrpc luns init.

Signed-off-by: Wenwei Tao <ww.tao0320@gmail.com>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>