LU-1187 lmv: allocate lmv tgts array by index

LU-1187 lmv: allocate lmv tgts array by index

Allocate lmv tgts array by MDT index, which is similar as lov
tgts arrary, so the index of tgts will be decided by MDT index,
instead of mount sequence in current implementation.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: If4903553251e1dba653aa79fb02bce581580629f
Reviewed-on: http://review.whamcloud.com/4936
Tested-by: Hudson
Reviewed-by: Fan Yong <fan.yong@intel.com>
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>