netlink: introduce typedef for filter function

Make the code using filter function a bit nicer by consolidating the
filter function arguments using typedef.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>