tg3: fix tigon3_dma_hwbug_workaround()

tg3: fix tigon3_dma_hwbug_workaround()

Ari got kernel panics using tg3 NIC, and bisected to 2669069aacc9 "tg3:
enable transmit time stamping."

This is because tigon3_dma_hwbug_workaround() might alloc a new skb and
free the original. We panic when skb_tx_timestamp() is called on freed
skb.

Reported-by: Ari Savolainen <ari.m.savolainen@gmail.com>
(cherry picked from commit f7ff19871bb4a3451e1ca2cf660bf633018cfbec)
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Joe Jin <joe.jin@oracle.com>