LU-5820 lfsck: use multiple namespace LFSCK trace files

LU-5820 lfsck: use multiple namespace LFSCK trace files

The namespace LFSCK uses trace file to record the FID of the object
that has multiple hard links, or has remote name entry, or contains
some uncertain inconsistency, and so on. Only single namespace LFSCK
trace file may be not efficient, especially when there are millions
of FIDs to be recorded. So use multiple namespace LFSCK trace files
and per trace file based semaphore to control the concurrent access
of the trace file.

For Lustre-2.x (x <= 6), the LFSCK used LFSCK_NAMESPACE_MAGIC_V1 as
the namespace trace file magic. When downgrade to such old release,
the old LFSCK will not recognize the new LFSCK_NAMESPACE_MAGIC_V2 in
the new trace file, then it will reset the whole LFSCK, and will not
cause start failure. The similar case will happen when upgrade from
such old release.

This patch also drops some repeated FID recording in the namespace
LFSCK trace file. Related FID should have been recorded in the trace
file via lfsck_namespace_exec_oit(), it is unnecessary to do that
again when scanning the directory.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: Iec27c52b21789dbde1e4c1153f61162f028ceac3
Reviewed-on: http://review.whamcloud.com/12809
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>