Currently, maybe_stop_tx ops for TSO and non-TSO case share some BD
calculation code, so this patch unifies the maybe_stop_tx by removing
the maybe_stop_tx ops. skb_is_gso() can be used to differentiate the
case between TSO and non-TSO case if there is need to handle special
case for TSO case.

This patch also add tx_copy field in "ethtool --statistics" to help
better debug the performance issue caused by calling skb_copy.
Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>