daemon/cluster: add a missing Unlock

Signed-off-by: Ziheng Liu <lzhfromustc@gmail.com>
(cherry picked from commit 83c0bedba914fa08c9d1f2fe1e47b38a3264777d)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
diff --git a/daemon/cluster/swarm.go b/daemon/cluster/swarm.go
index 727612c..9614528 100644
--- a/daemon/cluster/swarm.go
+++ b/daemon/cluster/swarm.go
@@ -33,6 +33,7 @@
 			// API handlers to finish before shutting down the node.
 			c.mu.Lock()
 			if !c.nr.nodeState.IsManager() {
+				c.mu.Unlock()
 				return "", errSwarmNotManager
 			}
 			c.mu.Unlock()