veth: use ethtool_sprintf instead of snprintf

use ethtools api ethtool_sprintf to instead of snprintf.

Signed-off-by: Tonghao Zhang <xiangxia.m.yue@gmail.com>
Link: https://lore.kernel.org/r/20211125025444.13115-1-xiangxia.m.yue@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>