lightnvm: pblk: recover only written metadata

This patch ensures that smeta was fully written before even
trying to read it based on chunk table state and write pointer.

Signed-off-by: Igor Konopko <igor.j.konopko@intel.com>
Signed-off-by: Matias Bjørling <mb@lightnvm.io>
Signed-off-by: Jens Axboe <axboe@kernel.dk>