ethtool: add LINKINFO_NTF notification

Send ETHTOOL_MSG_LINKINFO_NTF notification message whenever device link
settings are modified using ETHTOOL_MSG_LINKINFO_SET netlink message or
ETHTOOL_SLINKSETTINGS or ETHTOOL_SSET ioctl commands.

The notification message has the same format as reply to LINKINFO_GET
request. ETHTOOL_MSG_LINKINFO_SET netlink request only triggers the
notification if there is a change but the ioctl command handlers do not
check if there is an actual change and trigger the notification whenever
the commands are executed.

As all work is done by ethnl_default_notify() handler and callback
functions introduced to handle LINKINFO_GET requests, all that remains is
adding entries for ETHTOOL_MSG_LINKINFO_NTF into ethnl_notify_handlers and
ethnl_default_notify_ops lookup tables and calls to ethtool_notify() where
needed.

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>