add crcs for btree blocks

This patch series adds crcs for btree blocks. The first three patches contain some preparatory work, and the final one implements the actual extended btree blocks with crcs and a uuid / blkno pair to identify out of place writes. Note that this is only the start of CRC enabling XFS and it will be followed by more patches later.

To actually create a CRC enabled filesystem you will need a patched xfsprogsa . A quilt series with all required patches against current xfs-cmds CVS HEAD is here:

	http://verein.lst.de/~hch/xfs/xfs-cmds-crcs.tgz

Please note that the xfs-cmds patch series is not quite in a submittable shape yet, and will always create CRC enabled filesystems with these patches applied.