fuse: remove unneeded lock which protecting update of congestion_threshold

Commit 670d21c6 ("fuse: remove reliance on bdi congestion") change how
congestion_threshold is used and lock in
fuse_conn_congestion_threshold_write is not needed anymore.
1. Access to supe_block is removed along with removing of bdi congestion.
Then down_read(&fc->killsb) which protecting access to super_block is no
needed.
2. Compare num_background and congestion_threshold without holding
bg_lock. Then there is no need to hold bg_lock to update
congestion_threshold.
Signed-off-by: Kemeng Shi <shikemeng@huaweicloud.com>
Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>

fs/fuse/control.c

@@ -174,11 +174,7 @@ static ssize_t fuse_conn_congestion_threshold_write(struct file *file,
 	if (!fc)
 		goto out;
 
-	down_read(&fc->killsb);
-	spin_lock(&fc->bg_lock);
-	fc->congestion_threshold = val;
-	spin_unlock(&fc->bg_lock);
-	up_read(&fc->killsb);
+	WRITE_ONCE(fc->congestion_threshold, val);
 	fuse_conn_put(fc);
 out:
 	return ret;