md: raid5: avoid sector values going negative when testing reshape progress.

As sector_t in unsigned, we cannot afford to let 'safepos' etc go
negative.
So replace
   a -= b;
by
   a -= min(b,a);

Signed-off-by: NeilBrown <neilb@suse.de>
diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index 4616bc3..3c3626d 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -3811,13 +3811,13 @@
 	safepos = conf->reshape_safe;
 	sector_div(safepos, data_disks);
 	if (mddev->delta_disks < 0) {
-		writepos -= reshape_sectors;
+		writepos -= min(reshape_sectors, writepos);
 		readpos += reshape_sectors;
 		safepos += reshape_sectors;
 	} else {
 		writepos += reshape_sectors;
-		readpos -= reshape_sectors;
-		safepos -= reshape_sectors;
+		readpos -= min(reshape_sectors, readpos);
+		safepos -= min(reshape_sectors, safepos);
 	}
 
 	/* 'writepos' is the most advanced device address we might write.