xfs: fix rt device offset calculations for FITRIM

FITRIM on xfs has this bizarro uapi where we flatten all the physically
addressable storage across two block devices into a linear address
space.  In this address space, the realtime device comes immediately
after the data device.  Therefore, the xfs_trim_rtdev_extents has to
convert its input parameters from the linear address space to actual
rtdev block addresses on the realtime volume.

Right now the address space conversion is done in units of rtblocks.
However, a future patchset will convert xfs_rtblock_t to be a segmented
address space (group:blkno) like the data device.  Change the conversion
code to be done in units of daddrs since those will never be segmented.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Christoph Hellwig <hch@lst.de>

Author: Darrick J. Wong

fs/xfs/xfs_discard.c

@@ -480,7 +480,7 @@ xfs_discard_rtdev_extents(
 		trace_xfs_discard_rtextent(mp, busyp->bno, busyp->length);
 
 		error = __blkdev_issue_discard(bdev,
-				XFS_FSB_TO_BB(mp, busyp->bno),
+				xfs_rtb_to_daddr(mp, busyp->bno),
 				XFS_FSB_TO_BB(mp, busyp->length),
 				GFP_NOFS, &bio);
 		if (error)
@@ -612,22 +612,25 @@ xfs_trim_rtdev_extents(
 	xfs_rtblock_t		start_rtbno, end_rtbno;
 	xfs_rtxnum_t		start_rtx, end_rtx;
 	xfs_rgnumber_t		start_rgno, end_rgno;
+	xfs_daddr_t		daddr_offset;
 	int			last_error = 0, error;
 	struct xfs_rtgroup	*rtg = NULL;
 
 	/* Shift the start and end downwards to match the rt device. */
-	start_rtbno = xfs_daddr_to_rtb(mp, start);
-	if (start_rtbno > mp->m_sb.sb_dblocks)
-		start_rtbno -= mp->m_sb.sb_dblocks;
+	daddr_offset = XFS_FSB_TO_BB(mp, mp->m_sb.sb_dblocks);
+	if (start > daddr_offset)
+		start -= daddr_offset;
 	else
-		start_rtbno = 0;
+		start = 0;
+	start_rtbno = xfs_daddr_to_rtb(mp, start);
 	start_rtx = xfs_rtb_to_rtx(mp, start_rtbno);
 	start_rgno = xfs_rtb_to_rgno(mp, start_rtbno);
 
-	end_rtbno = xfs_daddr_to_rtb(mp, end);
-	if (end_rtbno <= mp->m_sb.sb_dblocks)
+	if (end <= daddr_offset)
 		return 0;
-	end_rtbno -= mp->m_sb.sb_dblocks;
+	else
+		end -= daddr_offset;
+	end_rtbno = xfs_daddr_to_rtb(mp, end);
 	end_rtx = xfs_rtb_to_rtx(mp, end_rtbno + mp->m_sb.sb_rextsize - 1);
 	end_rgno = xfs_rtb_to_rgno(mp, end_rtbno);