LU-12951 lmv: fix to return correct MDT count

LU-12951 lmv: fix to return correct MDT count

@ltd_tgts_size could be larger than actual MDT count,
as we preallocate ltd_tgts and resize it if necessary.

Fix it to use @ld_tgt_count instead.

Change-Id: I1501fd965cc74223c7a77280aac64acdbbcf17f6
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/36713
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-by: Olaf Faaland-LLNL <faaland1@llnl.gov>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>