Tx copybreak buf size is used for tx copybreak feature, the feature is
used for small size packet or frag. It adds a queue based tx shared
bounce buffer to memcpy the small packet when the len of xmitted skb is
below tx_copybreak(value to distinguish small size and normal size),
and reduce the overhead of dma map and unmap when IOMMU is on.

Support setting it via ethtool --set-tunable parameter and getting
it via ethtool --get-tunable parameter.
Signed-off-by: Hao Chen <chenhao288@hisilicon.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>