ethtool: add PRIVFLAGS_NTF notification

Send ETHTOOL_MSG_PRIVFLAGS_NTF notification whenever private flags of
a network device are modified using ETHTOOL_MSG_PRIVFLAGS_SET netlink
message or ETHTOOL_SPFLAGS ioctl request.

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Signed-off-by: David S. Miller <davem@davemloft.net>