LU-12960 lod: don't set index for 2nd stripe if specific

LU-12960 lod: don't set index for 2nd stripe if specific

When MDTs are specific, don't set index for the second stripe
allocation, otherwise it's not created on specific MDT.

Add sanity 31q.

Fixes: c1d0a355a6 ("LU-12624 lod: alloc dir stripes by QoS")
Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I415be0fa7c6e702ec72dcaac88bba55290463d44
Reviewed-on: https://review.whamcloud.com/36735
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>