xfs: set the refcount btree cowflag when setting up cow

xfs: set the refcount btree cowflag when setting up cow

When we're setting up a fake cow extent in the refcountbt to test
cleanup of fake cow extents, set the cowflag in the record field
to reflect our new disk format of storing the staging extents in
the right side of the tree.

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