LU-11681 lfsck: read LMV from bottom object

LU-11681 lfsck: read LMV from bottom object

LFSCK should read directory LMV from bottom object because it
doesn't want to read shard FIDs, and the allocated buffer size
can only contain struct lmv_mds_md_v1.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I2160b029ea4d19c5c056a0d06c340194a4811b00
Reviewed-on: https://review.whamcloud.com/33926
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Faccini Bruno <bruno.faccini@intel.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Fan Yong <fan.yong@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>