mm/vmscan: fix comment for current_may_throttle

Since commit 6d6435811c19 ("remove bdi_congested() and wb_congested() and
related functions"), there is no congested backing device check anymore. 
Correct the comment accordingly.

[akpm@linux-foundation.org: tweak grammar]
Link: https://lkml.kernel.org/r/20220414120202.30082-1-linmiaohe@huawei.com
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/vmscan.c

@@ -2297,10 +2297,9 @@ static unsigned int move_pages_to_lru(struct lruvec *lruvec,
 }
 
 /*
- * If a kernel thread (such as nfsd for loop-back mounts) services
+ * If a kernel thread (such as nfsd for loop-back mounts) services a backing
- * a backing device by writing to the page cache it sets PF_LOCAL_THROTTLE.
+ * device by writing to the page cache it sets PF_LOCAL_THROTTLE. In this case
- * In that case we should only throttle if the backing device it is
+ * we should not throttle.  Otherwise it is safe to do so.
- * writing to is congested.  In other cases it is safe to throttle.
  */
 static int current_may_throttle(void)
 {