xfs: add CRC checks to the AGI

Same set of changes made to the AGF need to be made to the AGI.
This patch has a similar history to the AGF, hence a similar
sign-off chain.

Signed-off-by: Dave Chinner <dgc@sgi.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dave Chinner <dgc@redhat.com>
Reviewed-by: Ben Myers <bpm@sgi.com>
Signed-off-by: Ben Myers <bpm@sgi.com>