r8169: improve rtl8169_start_xmit

r8169: improve rtl8169_start_xmit

Improve the following in rtl8169_start_xmit:
- tp->cur_tx can be accessed in parallel by rtl_tx(), therefore
  annotate the race by using WRITE_ONCE
- avoid checking stop_queue a second time by moving the doorbell check
- netif_stop_queue() uses atomic operation set_bit() that includes a
  full memory barrier on some platforms, therefore use
  smp_mb__after_atomic to avoid overhead

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://lore.kernel.org/r/80085451-3eaf-507a-c7c0-08d607c46fbc@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>