xfs: implement block-metadata based data checksums

xfs: implement block-metadata based data checksums

This is a quick hack to demonstrate how data checksumming can be
implemented when it can be stored in the out of line metadata for each
logical block.  It builds on top of the previous PI infrastructure
and instead of generating/verifying protection information it simply
generates and verifies a crc32c checksum and stores it in the non-PI
metadata.  It misses a feature bit in the superblock, checking that
enough size is available in the metadata and many other things.

Signed-off-by: Christoph Hellwig <hch@lst.de>