md: initialize percpu refcounters using PERCU_REF_ALLOW_REINIT

Percpu reference counters should now be initialized with the
PERCPU_REF_ALLOW_REINIT in order to allow switching them to the
percpu mode from the atomic mode.
To make percpu_ref_switch_to_percpu() call in set_in_sync()
succeed,let's initialize percpu refcounters with the
PERCU_REF_ALLOW_REINIT flag.

Signed-off-by: Roman Gushchin <guro@fb.com>
Acked-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Dennis Zhou <dennis@kernel.org>

drivers/md/md.c

@@ -5249,7 +5249,8 @@ int mddev_init_writes_pending(struct mddev *mddev)
 {
 	if (mddev->writes_pending.percpu_count_ptr)
 		return 0;
-	if (percpu_ref_init(&mddev->writes_pending, no_op, 0, GFP_KERNEL) < 0)
+	if (percpu_ref_init(&mddev->writes_pending, no_op,
+			    PERCPU_REF_ALLOW_REINIT, GFP_KERNEL) < 0)
 		return -ENOMEM;
 	/* We want to start with the refcount at zero */
 	percpu_ref_put(&mddev->writes_pending);