nfsd4: keep finer-grained callback status

Distinguish between when the callback channel is known to be down, and
when it is not yet confirmed.  This will be useful in the 4.1 case.

Also, we don't seem to be using the fact that this field is atomic.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>

fs/nfsd/state.h

@@ -242,7 +242,10 @@ struct nfs4_client {
 	unsigned long		cl_cb_flags;
 	struct rpc_clnt		*cl_cb_client;
 	u32			cl_cb_ident;
-	atomic_t		cl_cb_set;
+#define NFSD4_CB_UP		0
+#define NFSD4_CB_UNKNOWN	1
+#define NFSD4_CB_DOWN		2
+	int			cl_cb_state;
 	struct nfsd4_callback	cl_cb_null;
 	struct nfsd4_session	*cl_cb_session;