lightnvm: Make functions not used by ouside static

Mark functions not used by ouside of thier implementing file as static.

Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Matias Bjørling <m@bjorling.me>
Signed-off-by: Jens Axboe <axboe@fb.com>