f2fs: delay awaking discard thread

It's better to delay awaking discard thread while queuing discard commands
in checkpoint, it will help to give more chances for merging big and small
discard.
Signed-off-by: NChao Yu <yuchao0@huawei.com>
Signed-off-by: NJaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -968,7 +968,6 @@ static int __queue_discard_cmd(struct f2fs_sb_info *sbi,
 		blkstart -= FDEV(devi).start_blk;
 	}
 	__update_discard_tree_range(sbi, bdev, lblkstart, blkstart, blklen);
-	wake_up(&SM_I(sbi)->dcc_info->discard_wait_queue);
 	return 0;
 }
 
@@ -1335,6 +1334,8 @@ void clear_prefree_segments(struct f2fs_sb_info *sbi, struct cp_control *cpc)
 		SM_I(sbi)->dcc_info->nr_discards -= total_len;
 		kmem_cache_free(discard_entry_slab, entry);
 	}
+
+	wake_up(&SM_I(sbi)->dcc_info->discard_wait_queue);
 }
 
 static int create_discard_cmd_control(struct f2fs_sb_info *sbi)