nfsd4: lookup up callback cred only once

Lookup the callback cred once and then use it for all subsequent
callbacks.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>
diff --git a/fs/nfsd/nfs4state.c b/fs/nfsd/nfs4state.c
index 7e1fcc3..b205c7d 100644
--- a/fs/nfsd/nfs4state.c
+++ b/fs/nfsd/nfs4state.c
@@ -643,6 +643,10 @@
 		clp->cl_callback.cb_client = NULL;
 		rpc_shutdown_client(clnt);
 	}
+	if (clp->cl_callback.cb_cred) {
+		put_rpccred(clp->cl_callback.cb_cred);
+		clp->cl_callback.cb_cred = NULL;
+	}
 }
 
 static inline void