xfs_rtx_to_rtb
startblock = xfs_rtx_to_rtb(rtg, rec->ar_startext);
rtbno = xfs_rtx_to_rtb(rtg, rec->ar_startext);
rbno = xfs_rtx_to_rtb(rtg, rec->ar_startext);
xfs_rtx_to_rtb(rtg, rec->ar_startext);
*bno = xfs_rtx_to_rtb(args.rtg, rtx);