r8169: Fix possible ring buffer corruption on fragmented Tx packets.

An issue was found on the RTL8125b when transmitting small fragmented
packets, whereby invalid entries were inserted into the transmit ring
buffer, subsequently leading to calls to dma_unmap_single() with a null
address.

This was caused by rtl8169_start_xmit() not noticing changes to nr_frags
which may occur when small packets are padded (to work around hardware
quirks) in rtl8169_tso_csum_v2().

To fix this, postpone inspecting nr_frags until after any padding has been
applied.

Fixes: 9020845f ("r8169: improve rtl8169_start_xmit")
Cc: stable@vger.kernel.org
Signed-off-by: Ken Milmore <ken.milmore@gmail.com>
Reviewed-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://lore.kernel.org/r/27ead18b-c23d-4f49-a020-1fc482c5ac95@gmail.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>