net: bcmgenet: unshare the skb before writing the control block

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

net: bcmgenet: unshare the skb before writing the control block

bcmgenet stores first_cb, last_cb and bytes_sent in the skb control
block and reads them back on completion to free the skb and credit BQL.
ether_setup() leaves IFF_TX_SKB_SHARING set, so the stack can resubmit
a shared skb that is still in the ring. The next transmit overwrites
last_cb, so the earlier slots complete without crediting their bytes
and BQL stalls the queue until the watchdog resets it.

Unshare the skb before writing the control block.

Reproduce with pktgen in clone_skb mode, which hands the driver the same
skb while earlier copies are still in the ring:

modprobe pktgen
pg=/proc/net/pktgen
echo "add_device eth0" > $pg/kpktgend_0
echo "dst_mac AA:BB:CC:DD:EE:FF" > $pg/eth0
echo "dst 192.0.2.1" > $pg/eth0
echo "clone_skb 8" > $pg/eth0
echo "count 0" > $pg/eth0
echo start > $pg/pgctrl &
sleep 5
echo stop > $pg/pgctrl
wait
cat /sys/class/net/eth0/queues/tx-0/byte_queue_limits/inflight

inflight stays nonzero on the now idle queue. clone_skb 0 leaves it at
zero.