xfs: don't clobber inobt/finobt cursors when xref with rmap

Even if we can't use the inobt/finobt cursors to count the number of
inode btree blocks, we are never allowed to clobber the cursor of the
btree being checked, so don't do this.  Found by fuzzing level = ones
in xfs/364.

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