nfsd4: allow restarting callbacks

If we lose the backchannel and then the client repairs the problem,
resend any callbacks.

We use a new cb_done flag to track whether there is still work to be
done for the callback or whether it can be destroyed with the rpc.

Signed-off-by: J. Bruce Fields <bfields@redhat.com>
diff --git a/fs/nfsd/state.h b/fs/nfsd/state.h
index 4e5bdfd..3074656 100644
--- a/fs/nfsd/state.h
+++ b/fs/nfsd/state.h
@@ -68,10 +68,12 @@
 struct nfsd4_callback {
 	void *cb_op;
 	struct nfs4_client *cb_clp;
+	struct list_head cb_per_client;
 	u32 cb_minorversion;
 	struct rpc_message cb_msg;
 	const struct rpc_call_ops *cb_ops;
 	struct work_struct cb_work;
+	bool cb_done;
 };
 
 struct nfs4_delegation {
@@ -248,6 +250,7 @@
 	int			cl_cb_state;
 	struct nfsd4_callback	cl_cb_null;
 	struct nfsd4_session	*cl_cb_session;
+	struct list_head	cl_callbacks; /* list of in-progress callbacks */
 
 	/* for all client information that callback code might need: */
 	spinlock_t		cl_lock;