LU-2240 mds: Assign special fid sequence to root.

In current implementation, we still use IGIF as the ROOT FID,
but ZFS does not have IGIF at all, so it will move root ROOT fid
to a new sequence.

But for compatible with clients, which are being mounted
during upgrade, it will still keep the old IGIF root FID.

Signed-off-by: Wang Di <di.wang@intel.com>
Change-Id: I2e6b81cd359436d2e5b96c8a315a56c415f8e631
Reviewed-on: http://review.whamcloud.com/5257
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Mike Pershin <mike.pershin@intel.com>