ethtool: do not use rtnl in ethnl_default_dumpit()

for_each_netdev_dump() can be used with RCU protection,
no need for rtnl if we are going to use dev_hold()/dev_put().

Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20240207153514.3640952-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>