lightnvm: reserved space calculation incorrect

The nvm_dev->max_pages_per_blk variable was removed in favor of the new
nvm->sec_per_blk variable. The ->max_pages_per_blk variable was still
used in rrpc_capacity, reporting the reserved capacity to zero. Replace
with ->sec_per_blk to calculate the reserved area again.

Signed-off-by: Javier González <javier@cnexlabs.com>
Updated patch description. Was "lightnvm: eliminate redundant variable"
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>