ext2: correct max file size computing

We need to calculate the max file size accurately if the total blocks
that can address by block tree exceed the upper_limit. But this check is
not correct now, it only compute the total data blocks but missing
metadata blocks are needed. So in the case of "data blocks < upper_limit
&& total blocks > upper_limit", we will get wrong result. Fortunately,
this case could not happen in reality, but it's confused and better to
correct the computing.

  bits   data blocks   metadatablocks   upper_limit
  10        16843020            66051    2147483647
  11       134480396           263171    1073741823
  12      1074791436          1050627     536870911 (*)
  13      8594130956          4198403     268435455 (*)
  14     68736258060         16785411     134217727 (*)
  15    549822930956         67125251      67108863 (*)
  16   4398314962956        268468227      33554431 (*)

  [*] Need to calculate in depth.

Fixes: 1c2d142 ("ext2: Fix underflow in ext2_max_size()")
Link: https://lore.kernel.org/r/20220212050532.179055-1-yi.zhang@huawei.com
Signed-off-by: Zhang Yi <yi.zhang@huawei.com>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/ext2/super.c

@@ -753,8 +753,12 @@ static loff_t ext2_max_size(int bits)
 	res += 1LL << (bits-2);
 	res += 1LL << (2*(bits-2));
 	res += 1LL << (3*(bits-2));
+	/* Compute how many metadata blocks are needed */
+	meta_blocks = 1;
+	meta_blocks += 1 + ppb;
+	meta_blocks += 1 + ppb + ppb * ppb;
 	/* Does block tree limit file size? */
-	if (res < upper_limit)
+	if (res + meta_blocks <= upper_limit)
 		goto check_lfs;
 
 	res = upper_limit;