NFSv4.1: pass transport for callback shutdown

When we are setting up the 4.1 callback server, we pass in
the appropriate rpc_xprt transport pointer with which to associate
the callback server structure. Similarly, pass in the rpc_xprt
pointer for when we are shutting down the callback. This will be
used to make sure that we free the server structure and then clear
the rpc_xprt's bc_server pointer in a safe manner.

Signed-off-by: Olga Kornievskaia <[e-mail]>
Signed-off-by: Trond Myklebust <[e-mail]>

fs/nfs/callback.c

···
 /*
  * Kill the callback thread if it's no longer being used.
  */
-void nfs_callback_down(int minorversion, struct net *net)
+void nfs_callback_down(int minorversion, struct net *net, struct rpc_xprt *xprt)
 {
 	struct nfs_callback_data *cb_info = &nfs_callback_info[minorversion];
 	struct svc_serv *serv;

fs/nfs/callback.h

···
 				   struct cb_process_state *cps);
 #if IS_ENABLED(CONFIG_NFS_V4)
 extern int nfs_callback_up(u32 minorversion, struct rpc_xprt *xprt);
-extern void nfs_callback_down(int minorversion, struct net *net);
+extern void nfs_callback_down(int minorversion, struct net *net,
+			      struct rpc_xprt *xprt);
 #endif /* CONFIG_NFS_V4 */
 /*
  * nfs41: Callbacks are expected to not cause substantial latency,

fs/nfs/nfs4client.c

···
  */
 static void nfs4_destroy_callback(struct nfs_client *clp)
 {
-	if (__test_and_clear_bit(NFS_CS_CALLBACK, &clp->cl_res_state))
-		nfs_callback_down(clp->cl_mvops->minor_version, clp->cl_net);
+	if (__test_and_clear_bit(NFS_CS_CALLBACK, &clp->cl_res_state)) {
+		struct rpc_xprt *xprt;
+
+		xprt = rcu_dereference_raw(clp->cl_rpcclient->cl_xprt);
+		nfs_callback_down(clp->cl_mvops->minor_version, clp->cl_net,
+				  xprt);
+	}
 }
 
 static void nfs4_shutdown_client(struct nfs_client *clp)