xfs: use a xfs_btree_cur for the ->finish_cleanup state

Given how XFS is all based around btrees it doesn't make much sense
to offer a totally generic state when we can just use the btree cursor.

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>