Hello,

while trying to run a golang-ci linter on a macOS against an app using netlink.SocketDiagTCPInfo (added in #534), I'm getting a lint error that cannot be skipped:

undefined: netlink.SocketDiagTCPInfo (typecheck)

It seems that while many other functions have platform-agnostic code that returns ErrNotImplemented, this is not the case for many of the functions defined in socket_linux.go and socket_xdp_linux.go, except SocketGet and SocketDestroy (which live in netlink_unspecified.go).

To ease the linting, it would be useful to have the platform-agnostic socket functions defined too.