xfs_repair: complain about ag header crc errors

xfs_repair: complain about ag header crc errors

Repair doesn't complain about crc errors in the AG headers, and it
should.  Otherwise, this gives the admin the wrong impression about the
state of the filesystem after a nomodify check.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>
Reviewed-by: Allison Collins <allison.henderson@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Eric Sandeen <sandeen@sandeen.net>