LU-8301 lfsck: handle ROOT fid properly

It is found that the lfsck_find_mdt_idx_by_fid() will return
failure for "ROOT" object. That is incorrect. In fact "ROOT"
is always on the MDT0.

Test-Paramenters: trivial testlist=sanity-scrub,sanity-lfsck

Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I4a1b796ffc574f8f11611d1b329ce79ad2135eb7
Reviewed-on: http://review.whamcloud.com/20869
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Lai Siyao <lai.siyao@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>