repair: detect CRC errors in AG headers

repair: detect CRC errors in AG headers

repair doesn't currently detect verifier errors in AG header
blocks - apart from the primary superblock they are not detected.
They are, fortunately, corrected in the important cases (AGF, AGI
and AGFL) because these structures are rebuilt in phase 5, but if
you run xfs_repair in checking mode it won't report them as bad.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>