LU-7787 mdd: clean up orphan object handling

LU-7787 mdd: clean up orphan object handling

There was a potential problem in the orphan object naming because
it had an embedded space in the filename before the "operation",
which might cause issues if they are accessed for other reasons.
It turns out that there is no need for the "operation" to be
embedded into the filename, since it was always ORPH_OP_UNLINK.

Use standard DFID formatting for the orphan object names, which
is a bit shorter and more efficient on disk, without the embedded
operation type.

Remove the use of "ORPH_OP_UNLINK" in the code, except in the
compatibility code for handling orphans left over after upgrades
from older Lustre versions.  This can be removed at some point
in the future when there are no longer upgrades from pre-2.11
versions.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Change-Id: Ica90cc03c3212103c39cba11c4566584bf9cab07