nfsd: always release slot when requeueing callback

If the callback is going to be requeued to the workqueue, then release
the slot. The callback client and session could change and the slot may
no longer be valid after that point.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

fs/nfsd/nfs4callback.c

@@ -1405,6 +1405,7 @@ retry_nowait:
 	rpc_restart_call_prepare(task);
 	goto out;
 requeue:
+	nfsd41_cb_release_slot(cb);
 	nfsd4_requeue_cb(task, cb);
 	return false;
 }