lightnvm: pblk: set line bitmap check under debug

Do bitmap checks only when debug mode is enable. The line bitmap used
for mapping to physical addresses is fairly large (~512KB) and it is
expensive to do this checks on the fast path.

Signed-off-by: Javier González <javier@cnexlabs.com>
Signed-off-by: Matias Bjørling <matias@cnexlabs.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>