md/raid5: remove incorrect "min_t()" when calculating writepos.

This code is calculating:
  writepos, which is the furthest along address (device-space) that we
     *will* be writing to
  readpos, which is the earliest address that we *could* possible read
     from, and
  safepos, which is the earliest address in the 'old' section that we
     might read from after a crash when the reshape position is
     recovered from metadata.

  The first is a precise calculation, so clipping at zero doesn't
  make sense.  As the reshape position is now guaranteed to always be
  a multiple of reshape_sectors and as we already BUG_ON when
  reshape_progress is zero, there is no point in this min_t() call.

  The readpos and safepos are worst case - actual value depends on
  precise geometry.  That worst case could be negative, which is only
  a problem because we are storing the value in an unsigned.
  So leave the min_t() for those.
Signed-off-by: NeilBrown <neilb@suse.com>

drivers/md/raid5.c

@@ -5388,11 +5388,16 @@ static sector_t reshape_request(struct mddev *mddev, sector_t sector_nr, int *sk
 	safepos = conf->reshape_safe;
 	sector_div(safepos, data_disks);
 	if (mddev->reshape_backwards) {
-		writepos -= min_t(sector_t, reshape_sectors, writepos);
+		BUG_ON(writepos < reshape_sectors);
+		writepos -= reshape_sectors;
 		readpos += reshape_sectors;
 		safepos += reshape_sectors;
 	} else {
 		writepos += reshape_sectors;
+		/* readpos and safepos are worst-case calculations.
+		 * A negative number is overly pessimistic, and causes
+		 * obvious problems for unsigned storage.  So clip to 0.
+		 */
 		readpos -= min_t(sector_t, reshape_sectors, readpos);
 		safepos -= min_t(sector_t, reshape_sectors, safepos);
 	}