ethtool: push the rtnl_lock into dev_ethtool()

Don't take the lock in net/core/dev_ioctl.c,
we'll have things to do outside rtnl_lock soon.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>