ethtool: add support to set/get rx buf len via ethtool

Add support to set rx buf len via ethtool -G parameter and get
rx buf len via ethtool -g 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>