fs/ext4: Fix group descriptor checksum calculation

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>