md/raid1: fix bug in read_balance introduced by hot-replace

When we added hot_replace we doubled the number of devices
that could be in a RAID1 array.  So we doubled how far read_balance
would search.  Unfortunately we didn't double the point at which
it looped back to the beginning - so it effectively loops over
all non-replacement disks twice.
This doesn't cause bad behaviour, but it pointless and means we
never read from replacement devices.
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid1.c

@@ -517,8 +517,8 @@ static int read_balance(struct r1conf *conf, struct r1bio *r1_bio, int *max_sect
 		int bad_sectors;
 
 		int disk = start_disk + i;
-		if (disk >= conf->raid_disks)
-			disk -= conf->raid_disks;
+		if (disk >= conf->raid_disks * 2)
+			disk -= conf->raid_disks * 2;
 
 		rdev = rcu_dereference(conf->mirrors[disk].rdev);
 		if (r1_bio->bios[disk] == IO_BLOCKED