NEX-3079 port illumos ARC improvements

*** 651,661 ****
          if (flags & ZINJECT_FLUSH_ARC)
                  /*
                   * We must use FALSE to ensure arc_flush returns, since
                   * we're not preventing concurrent ARC insertions.
                   */
!                 arc_flush(NULL, FALSE);
  
          return (0);
  }
  
  /*
--- 651,661 ----
          if (flags & ZINJECT_FLUSH_ARC)
                  /*
                   * We must use FALSE to ensure arc_flush returns, since
                   * we're not preventing concurrent ARC insertions.
                   */
!                 arc_flush(NULL, B_FALSE);
  
          return (0);
  }
  
  /*