LU-2793 fld: send special seq lookup request to MDT0

LU-2793 fld: send special seq lookup request to MDT0

Since almost all special sequence locates on MDT0, we should
send all seq lookup req to MDT0, especially in an evironment
where other MDTs might not be started at all.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: Ifda9cb434a217d1e54dc2ef4fcb7628fca049d9d
Reviewed-on: http://review.whamcloud.com/5319
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>