lightnvm: remove struct nvm_dev->total_blocks

The struct nvm_dev->total_blocks was only used for calculating total
sectors. Remove and instead calculate total sectors from the number of
luns and its sectors.

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