Updated to use *_2_0_MDT_STATS for BACKFS_ZFS

The current Lustre-Orion code uses the same "md_stats" file for it's MDT
statistics as Lustre 2.0; no longer using the Lustre 1.8 "stats" file.
This patch updates _build_osd_stats_path to account for this change.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Closes #13