xfs: update got in xfs_bmap_shift_update_extent

This way the caller gets the proper updated extent returned in got.

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>