This is prepared for the further call back usage. Change-Id: Iccf304c87e67debfb4e7c330acc9cc233cc3ec48 Signed-off-by: Ziye Yang <ziye.yang@intel.com> Reviewed-on: https://review.gerrithub.io/c/spdk/spdk/+/481917 Community-CI: SPDK CI Jenkins <sys_sgci@intel.com> Tested-by: SPDK CI Jenkins <sys_sgci@intel.com> Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com> Reviewed-by: Ben Walker <benjamin.walker@intel.com> Reviewed-by: Jim Harris <james.r.harris@intel.com>