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

In the Linux kernel, the following vulnerability has been resolved: 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.