As 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 and use a helper
xfs_ag_shrink_space() to fixup the last AG.

This can be all done in one transaction for now, so I think no
additional protection is needed.
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Gao Xiang <hsiangkao@redhat.com>
Signed-off-by: Darrick J. Wong <djwong@kernel.org>