net: xilinx: axienet: Fix BQL accounting for multi-BD TX packets

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

net: xilinx: axienet: Fix BQL accounting for multi-BD TX packets

When a TX packet spans multiple buffer descriptors (scatter-gather),

axienet_free_tx_chain sums the per-BD actual length from descriptor

status into a caller-provided accumulator. That sum is reset on each

NAPI poll. If the BDs for a single packet complete across different

polls, the earlier bytes are lost and never credited to BQL. This

causes BQL to think bytes are permanently in-flight, eventually

stalling the TX queue.

The SKB pointer is stored only on the last BD of a packet. When that

BD completes, use skb->len for the byte count instead of summing

per-BD status lengths. This matches netdev_sent_queue(), which debits

skb->len, and naturally survives across polls because no partial

packet contributes to the accumulator.