xfs: use rtrefcount btree cursor in xchk_xref_is_rt_cow_staging

xfs: use rtrefcount btree cursor in xchk_xref_is_rt_cow_staging

LOLLM points out that we pass the wrong btree cursor here.  We want the
rtrefcount btree cursor, not the non-rt one.  This is fairly benign
since it only affects tracing data.

Cc: stable@vger.kernel.org # v6.14
Fixes: 91683bb3f264c0 ("xfs: cross-reference checks with the rt refcount btree")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Assisted-by: LOLLM # finding obvious bugs
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Carlos Maiolino <cem@kernel.org>