f2fs: Fix the accounting of dcc->undiscard_blks

When a discard_cmd needs to be split due to dpolicy->max_requests, then
for the remaining length it will be either merged into another cmd or a
new discard_cmd will be created. In this case, there is double
accounting of dcc->undiscard_blks for the remaining len, due to which
it shows incorrect value in stats.
Signed-off-by: Sahitya Tummala <stummala@codeaurora.org>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/segment.c

@@ -1214,8 +1214,10 @@ static int __submit_discard_cmd(struct f2fs_sb_info *sbi,
 		len = total_len;
 	}
 
-	if (!err && len)
+	if (!err && len) {
+		dcc->undiscard_blks -= len;
 		__update_discard_tree_range(sbi, bdev, lstart, start, len);
+	}
 	return err;
 }