xfs: allow unaligned extent records in xfs_bmbt_disk_set_all

To make life a little simpler make xfs_bmbt_set_all unaligned access
aware so that we can use it directly on the destination buffer.

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