net: caif: clear client service pointer on teardown

In the Linux kernel, the following vulnerability has been resolved: net: caif: clear client service pointer on teardown `caif_connect()` can tear down an existing client after remote shutdown by calling `caif_disconnect_client()` followed by `caif_free_client()`. `caif_free_client()` releases the service layer referenced by `adap_layer->dn`, but leaves that pointer stale. When the socket is later destroyed, `caif_sock_destructor()` calls `caif_free_client()` again and dereferences the freed service pointer. Clear the client/service links before releasing the service object so repeated teardown becomes harmless.