LU-2856 mdt: Workaround for filenames w/zero hash

Zero is a valid return value of full_name_hash(), but
consumers of mlh_pdo_hash don't handle this case correctly.
Work around problems by consistently mapping zero hashes to
a different value.

Signed-off-by: Daniel Kobras <d.kobras@science-computing.de>
Change-Id: I8b2622cb9b04d9f005f8e414ad35102155820c7d
Reviewed-on: http://review.whamcloud.com/6166
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>