xfs: fix backwards mergeability logic in refcount scrubber

When we start the refcount or rtrefcount btree scanners, prev_rec is
initialized to all zeroes.  This is done so that the record mergeability
checks skip the first record because you must have two records to
compare.  Unfortunately, I got the logic backwards, so scrub has never
complained about mergeable refcountbt records.  Fix this bug that LOLLM
noticed.

Cc: <stable@vger.kernel.org> # v6.4
Fixes: db0502b39c21d1 ("xfs: flag refcount btree records that could be merged")
Signed-off-by: "Darrick J. Wong" <djwong@kernel.org>
Assisted-by: LOLLM # finding obvious bugs
Reviewed-by: Christoph Hellwig <hch@lst.de>