ceph-disk: fix list for encrypted or corrupt volume

Continue gracefully if an fs type is not detected, either because it is
encrypted or because it is corrupted.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1778 from ceph/wip-7157

ceph-disk: fix list for encrypted or corrupt volume

Reviewed-by: Alfredo Deza <alfredo.deza@inktank.com>