xfs: only cancel cow blocks when truncating the data fork

In xfs_itruncate_extents, only cancel cow blocks and clear the reflink
flag if we were asked to truncate the data fork.  Attr fork blocks
cannot be shared, so this makes no sense.

Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>