ethtool: set coalescing parameters with COALESCE_SET request

Implement COALESCE_SET netlink request to set coalescing parameters of
a network device. These are traditionally set with ETHTOOL_SCOALESCE ioctl
request. This commit adds only support for device coalescing parameters,
not per queue coalescing parameters.

Like the ioctl implementation, the generic ethtool code checks if only
supported parameters are modified; if not, first offending attribute is
reported using extack.

v2: fix alignment (whitespace only)

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