xfs: pass struct xfs_bmbt_irec to xfs_bmbt_validate_extent

This removed an unaligned load per extent, as well as the manual poking
into the on-disk extent format.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>