LU-6320 lod: Use target index as the master index

LU-6320 lod: Use target index as the master index

Because lum_stripe_offset might be -1, if it comes
from the default stripe, and also the master stripe
always in the same MDT as the master object, so
it should use master object MDT index as the master
stripe index.

Signed-off-by: wang di <di.wang@intel.com>
Change-Id: I4a700580dd7a035b243d2314231e50e12742100e
Reviewed-on: http://review.whamcloud.com/13941
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: John L. Hammond <john.hammond@intel.com>