LU-11025 lod: refactor lod_mdt_alloc_qos/rr()

LU-11025 lod: refactor lod_mdt_alloc_qos/rr()

Add arguments 'stripe_idx' and 'stripe_count' to
lod_mdt_alloc_qos/rr(), so later directory split/merge can allocate
stripes from specified stripe index.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Id054ac397fd632228fa6c2786c90e5aad9aa2eb1
Reviewed-on: https://review.whamcloud.com/36865
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>