LU-13357 lod: implement striped directory .dio_lookup

LU-13357 lod: implement striped directory .dio_lookup

Add function lod_striped_lookup() for
lod_striped_index_ops.dio_lookup to allow name lookup under striped
directory.

Currently this is used by subdir mount, which needs to lookup FID
of the subdir on server side.

Function lfsck_namespace_repair_dirent() should call dt_lookup() with
bottom object, because child may be shard.

Add sanity 247f.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Iba844d1a34a318bcbd42b00186ed6fa9d165effc
Reviewed-on: https://review.whamcloud.com/37903
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>