ethtool: provide coalescing parameters with COALESCE_GET request

Implement COALESCE_GET request to get coalescing parameters of a network
device. These are traditionally available via ETHTOOL_GCOALESCE ioctl
request. This commit adds only support for device coalescing parameters,
not per queue coalescing parameters.

Omit attributes with zero values unless they are declared as supported
(i.e. the corresponding bit in ethtool_ops::supported_coalesce_params is
set).

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