xfs: fix scrub tracepoints when inode-rooted btrees are involved

Fix a minor mistakes in the scrub tracepoints that can manifest when
inode-rooted btrees are enabled.  The existing code worked fine for bmap
btrees, but we should tighten the code up to be less sloppy.

Cc: <stable@vger.kernel.org> # v5.7
Fixes: 92219c29 ("xfs: convert btree cursor inode-private member names")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>