xfs: support shrinking unused space in the last AG

At the first step of shrinking, this attempts to enable shrinking
unused space in the last allocation group by fixing up freespace
btree, agi, agf and adjusting super block.

This can be all done in one transaction for now, so I think no
additional protection is needed.

Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
6 files changed