LU-6321 lfsck: make lfsck_namespace trace file as index

LU-6321 lfsck: make lfsck_namespace trace file as index

Originally, the "lfsck_namespace" file stored both the namespace
LFSCK statistics information and the FIDs to be double scanned.
But to improve the namespace LFSCK performance (since Lustre-2.7),
we used multiple trace files with the name "lfsck_namespace_xx".
At that time, the original "lfsck_namespace" file only need to
record the namespace LFSCK statistics information. So we made it
as regular file, NOT index file. Such changes will cause trouble
when downgrade to Lustre-2.6 or older, becuase the old namespace
LFSCK needs an index trace file instead of regular file. To avoid
the compatibility issues, we will keep the "lfsck_namespace" file
as index file on b2_7 and newer release.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I76d8b1416c4c507793aa9bbab2d52cc7d8daa440
Reviewed-on: http://review.whamcloud.com/13945
Tested-by: Jenkins
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>