xfs: remove xfs_reflink_dirty_extents

Now that xfs_file_unshare is not completely dumb we can just call it
directly without iterating the extent and reflink btrees ourselves.

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>