MFC: r333879, r342749

• Even though 64-bit atomics are supported on i386 there are panics indicating that the code does not work correctly there. Switch to mutex based variant (and fix that while we're here). Reported by: pho, kib

• mp_ring: avoid items offset difference between iflib and mp_ring on architectures without 64-bit atomics Reported by: Augustin Cavalier <waddlesplash@gmail.com>