xfs: minor cleanup for xfs_reflink_end_cow

Use xfs_iext_prev_extent to skip to the previous extent instead of
opencoding it.

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>