Make IO channel flush call after the inflight request has been drained
in multifd thread, or else we may missed to flush the inflight request.
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@intel.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>