lightnvm: pblk: reduce L2P memory footprint

Currently L2P map size is calculated based on the total number of
available sectors, which is redundant, since it contains mapping for
overprovisioning as well (11% by default).

Change this size to the real capacity and thus reduce the memory
footprint significantly - with default op value it is approx.
110MB of DRAM less for every 1TB of media.

Signed-off-by: Igor Konopko <igor.j.konopko@intel.com>
Reviewed-by: Hans Holmberg <hans.holmberg@cnexlabs.com>
Reviewed-by: Javier González <javier@javigon.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>