netlink: add support for NIC driver ioctls

By returning -ENOIOCTLCMD, sock_do_ioctl() falls back to calling
dev_ioctl(), which provides support for NIC driver ioctls, which
includes ethtool support. This is similar to the way ioctls are handled
in udp.c or tcp.c.

This removes the requirement that ethtool for example be tied to the
support of a specific L3 protocol (ethtool uses an AF_INET socket
today).

Signed-off-by: David Decotigny <decot@googlers.com>
Signed-off-by: David S. Miller <davem@davemloft.net>