lightnvm: pblk: ensure that erase is chunk aligned

The sector bits in the erase command may be uninitialized are
uninitialized, causing the erase LBA to be unaligned to the chunk size.

This is unexpected situation, since erase shall always be chunk
aligned based on OCSSD the 2.0 specification.

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>