fix DBUG_RETURN usage.
fix DBUG_RETURN usage.

DBUG_RETURN(function()) should not be used because it logs
"<MESSAGE" before function() call. It causes wrong trace indent:

  int ha_mroonga::a()
  {
    MRN_DBUG_ENTER_METHOD();
    DBUG_RETURN(0);
  }

  int ha_mroonga::b()
  {
    MRN_DBUG_ENTER_METHOD();
    DBUG_RETURN(a());
  }

logs

  >ha_mroonga::b
  <ha_mroonga::b
    >ha_mroonga::a
    <ha_mroonga::a

It should be

  int ha_mroonga::a()
  {
    MRN_DBUG_ENTER_METHOD();
    DBUG_RETURN(0);
  }

  int ha_mroonga::b()
  {
    MRN_DBUG_ENTER_METHOD();
    int result = a();
    DBUG_RETURN(result);
  }

It logs

  >ha_mroonga::b
    >ha_mroonga::a
    <ha_mroonga::a
  <ha_mroonga::b

yoku0825/mroonga,yoku0825/mroonga,naoa/mroonga,yoku0825/mroonga,mroonga/mroonga,yoku0825/mroonga,naoa/mroonga,naoa/mroonga,mroonga/mroonga,yoku0825/mroonga,naoa/mroonga,naoa/mroonga,yoku0825/mroonga,naoa/mroonga,mroonga/mroonga,mroonga/mroonga