ethtool: provide link mode information with LINKMODES_GET request

Implement LINKMODES_GET netlink request to get link modes related
information provided by ETHTOOL_GLINKSETTINGS and ETHTOOL_GSET ioctl
commands.

This request provides supported, advertised and peer advertised link modes,
autonegotiation flag, speed and duplex.

LINKMODES_GET request can be used with NLM_F_DUMP (without device
identification) to request the information for all devices in current
network namespace providing the data.

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>