ethtool: add COALESCE_NTF notification

Send ETHTOOL_MSG_COALESCE_NTF notification whenever coalescing parameters
of a network device are modified using ETHTOOL_MSG_COALESCE_SET netlink
message or ETHTOOL_SCOALESCE ioctl request.

Signed-off-by: Michal Kubecek <mkubecek@suse.cz>
Reviewed-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>