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 <stefanha@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

include/net/af_vsock.h

@@ -165,6 +165,9 @@ static inline int vsock_core_init(const struct vsock_transport *t)
 }
 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

@@ -1995,6 +1995,15 @@ void vsock_core_exit(void)
 }
 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");