LU-15832 lod: clear .do_index_ops in striping free

LU-15832 lod: clear .do_index_ops in striping free

LDLM lock can guarantee LOD object directory striping is safe to
access, but lod_striping_free_nolock() should clear .do_index_ops,
otherwise upon some failure the directory striping is freed, while a
subsequent dt_try_as_dir() skips striping initialization, and call
.do_index_ops directly, which will cause crash.

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: Ib94a4ef2f8bf5f0d34521abff77d8be46ecbf428
Reviewed-on: https://review.whamcloud.com/47256
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>