md/raid10: Fix bug when activating a hot-spare.

This is a fairly serious bug in RAID10.

When a RAID10 array is degraded and a hot-spare is activated, the
spare does not take up the empty slot, but rather replaces the first
working device.
This is likely to make the array non-functional.   It would normally
be possible to recover the data, but that would need care and is not
guaranteed.

This bug was introduced in commit
   2bb77736ae5dca0a189829fbb7379d43364a9dac
which first appeared in 3.1.

Cc: stable@kernel.org
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid10.c

@@ -1355,7 +1355,7 @@ static int raid10_add_disk(struct mddev *mddev, struct md_rdev *rdev)
 		struct mirror_info *p = &conf->mirrors[mirror];
 		if (p->recovery_disabled == mddev->recovery_disabled)
 			continue;
-		if (!p->rdev)
+		if (p->rdev)
 			continue;
 
 		disk_stack_limits(mddev->gendisk, rdev->bdev,