Btrfs: clean up for wait_extent_bit()

We can just use cond_resched_lock().

Signed-off-by: Xiao Guangrong <xiaoguangrong@cn.fujitsu.com>
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Signed-off-by: Chris Mason <chris.mason@oracle.com>

fs/btrfs/extent_io.c

@@ -648,11 +648,7 @@ again:
 		if (start > end)
 			break;
 
-		if (need_resched()) {
-			spin_unlock(&tree->lock);
-			cond_resched();
-			spin_lock(&tree->lock);
-		}
+		cond_resched_lock(&tree->lock);
 	}
 out:
 	spin_unlock(&tree->lock);