fs/ext4: Fix group descriptor checksum calculation

The current code doesn't compute the group descriptor checksum correctly
for the filesystems that e2fsprogs 1.43.4 creates (they have
'Group descriptor size: 64' as reported by tune2fs). Extend the checksum
calculation to be done as ext4_group_desc_csum() does in Linux.

This fixes these errors in dmesg from running fs-test.sh and makes it
succeed again:

[1671902.620699] EXT4-fs (loop1): ext4_check_descriptors: Checksum for group 0 failed (35782!=10965)
[1671902.620706] EXT4-fs (loop1): group descriptors corrupted!

Signed-off-by: Tuomas Tynkkynen <tuomas.tynkkynen@iki.fi>

fs/ext4/ext4_common.c

@@ -432,6 +432,10 @@ uint16_t ext4fs_checksum_update(uint32_t i)
 		crc = ext2fs_crc16(crc, desc, offset);
 		offset += sizeof(desc->bg_checksum);	/* skip checksum */
 		assert(offset == sizeof(*desc));
+		if (offset < fs->gdsize) {
+			crc = ext2fs_crc16(crc, (__u8 *)desc + offset,
+					   fs->gdsize - offset);
+		}
 	}
 
 	return crc;