LU-2802 lmv: get tgt by checking ltd_idx in lmv_find_target.

Currently, lmv_find_target return lmv->tgts[mds]
according to mdt_index, which is not correct. LMV
index is created by mount sequence, while mdt_index
by indicated by --index. So we should check ltd_idx
in lmv_find_target.

Signed-off-by: Wang Di <di.wang@intel.com>
Change-Id: I67a941cca00eb80ba91af6eb3f3441982d4fcab3
Reviewed-on: http://review.whamcloud.com/5412
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>