xfs: use the rt version of the cow staging checker

xfs: use the rt version of the cow staging checker

LOLLM also noticed that xchk_rtrmapbt_xref ought to be using the rtdev
version of the "is this a cow extent?" helper function, not the datadev
one.

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>