New checksum 3.5.x kernel

Noticed that 3.5.x kernel uses the following flag:
EXT4_FEATURE_RO_COMPAT_METADATA_CSUM

And uses this function with a super block seed value to compute checksums over various structures:
ext4_chksum

As the newer version of the kernel works its way into regular usage, this change will need to be accounted for.
