staging: lustre: lmv: honor MDT index when creating volatile file

LMV should honor MDT index embedded in the name of volatile
file, then during hsm restore, the file under striped dir can
be restored to the right MDT.

Signed-off-by: wang di <di.wang@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-4691
Reviewed-on: http://review.whamcloud.com/10866


Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Henri Doreau <henri.doreau@cea.fr>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>