LU-11279 lod: reset ostlist properly in lod_get_default_lov_striping

LU-11279 lod: reset ostlist properly in lod_get_default_lov_striping

Ostlist might be allocated previously, and we should
reset them properly, otherwise, it will pollute new
default setting and cause unexpected behavior.

Test-Parameters: trivial testlist=sanity,sanity,sanity,sanity,sanity,sanity,sanity,sanity,sanity
Change-Id: I9b7acb5f05ec4b371da99f68b9647f0b75cd7021
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/33069
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>