VSOCK: transport-specific vsock_transport functions

struct vsock_transport contains function pointers called by AF_VSOCK
core code. The transport may want its own transport-specific function
pointers and they can be added after struct vsock_transport.

Allow the transport to fetch vsock_transport. It can downcast it to
access transport-specific function pointers.

The virtio transport will use this.

Signed-off-by: Stefan Hajnoczi <[e-mail]>
Signed-off-by: Michael S. Tsirkin <[e-mail]>

include/net/af_vsock.h

···
 }
 void vsock_core_exit(void);
 
+/* The transport may downcast this to access transport-specific functions */
+const struct vsock_transport *vsock_core_get_transport(void);
+
 /**** UTILS ****/
 
 void vsock_release_pending(struct sock *pending);

net/vmw_vsock/af_vsock.c

···
 }
 EXPORT_SYMBOL_GPL(vsock_core_exit);
 
+const struct vsock_transport *vsock_core_get_transport(void)
+{
+	/* vsock_register_mutex not taken since only the transport uses this
+	 * function and only while registered.
+	 */
+	return transport;
+}
+EXPORT_SYMBOL_GPL(vsock_core_get_transport);
+
 MODULE_AUTHOR("VMware, Inc.");
 MODULE_DESCRIPTION("VMware Virtual Socket Family");
 MODULE_VERSION("1.0.1.0-k");