LU-5707 lfsck: store namespace LFSCK statistics info in new EA

LU-5707 lfsck: store namespace LFSCK statistics info in new EA

For Lustre-2.6 or older release, the namespace LFSCK statistics info
was stored as XATTR_NAME_LFSCK_NAMESPACE EA, but in Lustre-2.7, the
namespace LFSCK will introduce more statistics information that will
cause the XATTR_NAME_LFSCK_NAMESPACE EA to be extended. If it still
uses the old XATTR_NAME_LFSCK_NAMESPACE EA, then when downgrade, the
old LFSCK will get -ERANGE when load the new trace file from disk,
and then the LFSCK cannot be started after downgrade.

To avoid such trouble, Lustre-2.7 will use new EA to store the
namespace LFSCK statistics info: XATTR_NAME_LFSCK_NAMESPACE_V2,
and keep a dummy XATTR_NAME_LFSCK_NAMESPACE EA in the trace file
to be compatible with old LFSCK.

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I55b5adb962434013b00e3938a67b671010ecc206
Reviewed-on: http://review.whamcloud.com/12321
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>