xfs: return the converted extent in __xfs_reflink_convert_cow

We'll need it for the direct I/O code.  Also rename the function to
xfs_reflink_convert_cow_extent to describe it a bit better.

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>