ext4: fix loff_t overflow in ext4_max_bitmap_size()

We should use unsigned long long rather than loff_t to avoid
overflow in ext4_max_bitmap_size() for comparison before returning.
w/o this patch sbi->s_bitmap_maxbytes was becoming a negative
value due to overflow of upper_limit (with has_huge_files as true)

Below is a quick test to trigger it on a 64KB pagesize system.

sudo mkfs.ext4 -b 65536 -O ^has_extents,^64bit /dev/loop2
sudo mount /dev/loop2 /mnt
sudo echo "hello" > /mnt/hello 	-> This will error out with
				"echo: write error: File too large"
Signed-off-by: Ritesh Harjani <riteshh@linux.ibm.com>
Reviewed-by: Jan Kara <jack@suse.cz>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Cc: stable@kernel.org
Link: https://lore.kernel.org/r/594f409e2c543e90fd836b78188dfa5c575065ba.1622867594.git.riteshh@linux.ibm.comSigned-off-by: Theodore Ts'o <tytso@mit.edu>

fs/ext4/super.c

@@ -3030,17 +3030,17 @@ static loff_t ext4_max_size(int blkbits, int has_huge_files)
  */
 static loff_t ext4_max_bitmap_size(int bits, int has_huge_files)
 {
-	loff_t res = EXT4_NDIR_BLOCKS;
+	unsigned long long upper_limit, res = EXT4_NDIR_BLOCKS;
 	int meta_blocks;
-	loff_t upper_limit;
-	/* This is calculated to be the largest file size for a dense, block
+
+	/*
+	 * This is calculated to be the largest file size for a dense, block
 	 * mapped file such that the file's total number of 512-byte sectors,
 	 * including data and all indirect blocks, does not exceed (2^48 - 1).
 	 *
 	 * __u32 i_blocks_lo and _u16 i_blocks_high represent the total
 	 * number of 512-byte sectors of the file.
 	 */
-
 	if (!has_huge_files) {
 		/*
 		 * !has_huge_files or implies that the inode i_block field
@@ -3083,7 +3083,7 @@ static loff_t ext4_max_bitmap_size(int bits, int has_huge_files)
 	if (res > MAX_LFS_FILESIZE)
 		res = MAX_LFS_FILESIZE;
 
-	return res;
+	return (loff_t)res;
 }
 
 static ext4_fsblk_t descriptor_loc(struct super_block *sb,