lightnvm: pblk: propagate errors when reading meta

Read errors are not correctly propagated. Errors are cleared before
returning control to the io submitter. Change the behaviour such that
all read errors exept high ecc read warning status is returned
appropriately.

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