md: always set THREAD_WAKEUP and wake up wqueue if thread existed

Since commit 4ad23a97 ("MD: use per-cpu counter for writes_pending"),
the wait_queue is only got invoked if THREAD_WAKEUP is not set previously.

With above change, I can see process_metadata_update could always hang on
the wait queue, because mddev->thread could stay on 'D' status and the
THREAD_WAKEUP flag is not cleared since there are lots of place to wake up
mddev->thread. Then deadlock happened as follows:

linux175:~ # ps aux|grep md|grep D
root    20117   0.0 0.0         0   0 ? D   03:45   0:00 [md0_raid1]
root    20125   0.0 0.0         0   0 ? D   03:45   0:00 [md0_cluster_rec]
linux175:~ # cat /proc/20117/stack
[<ffffffffa0635604>] dlm_lock_sync+0x94/0xd0 [md_cluster]
[<ffffffffa0635674>] lock_token+0x34/0xd0 [md_cluster]
[<ffffffffa0635804>] metadata_update_start+0x64/0x110 [md_cluster]
[<ffffffffa04d985b>] md_update_sb.part.58+0x9b/0x860 [md_mod]
[<ffffffffa04da035>] md_update_sb+0x15/0x30 [md_mod]
[<ffffffffa04dc066>] md_check_recovery+0x266/0x490 [md_mod]
[<ffffffffa06450e2>] raid1d+0x42/0x810 [raid1]
[<ffffffffa04d2252>] md_thread+0x122/0x150 [md_mod]
[<ffffffff81091741>] kthread+0x101/0x140
linux175:~ # cat /proc/20125/stack
[<ffffffffa0636679>] recv_daemon+0x3f9/0x5c0 [md_cluster]
[<ffffffffa04d2252>] md_thread+0x122/0x150 [md_mod]
[<ffffffff81091741>] kthread+0x101/0x140

So let's revert the part of code in the commit to resovle the problem since
we can't get lots of benefits of previous change.

Fixes: 4ad23a97 ("MD: use per-cpu counter for writes_pending")
Signed-off-by: Guoqing Jiang <gqjiang@suse.com>
Signed-off-by: Shaohua Li <shli@fb.com>

drivers/md/md.c

@@ -7468,7 +7468,7 @@ void md_wakeup_thread(struct md_thread *thread)
 {
 	if (thread) {
 		pr_debug("md: waking up MD thread %s.\n", thread->tsk->comm);
-		if (!test_and_set_bit(THREAD_WAKEUP, &thread->flags))
+		set_bit(THREAD_WAKEUP, &thread->flags);
 		wake_up(&thread->wqueue);
 	}
 }