md currently uses csum_partial to calculate checksums for superblocks.
However this function is not consistent across all architectures.  Some
(i386) to a 32bit csum.  Some (alpha) do a 16 bit csum.  This makes it hard
for userspace to keep up.

So we provide a generic routine (that does exactly what the i386
csum_partial does) and:

- When setting the csum, use csum_partial so that old kernels will still
  recognise the superblock

- When checking the csum, allow either csum_partial or the new generic
  code to provide the right csum.  This allows user-space to just use the
  common code and always work.

Also modify the csum for version-1 superblock (which currently aren't being
used) to always user a predictable checksum algorithm.

Thanks to Mike Tran <mhtran@us.ibm.com> for noticing this.
Signed-off-by: Neil Brown <neilb@cse.unsw.edu.au>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>