md: Fix bug where spares don't always get rebuilt properly when they become live

If save_raid_disk is >= 0, then the device could be a device that is
already in sync that is being re-added.  So we need to default this
value to -1.
Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Adrian Bunk <bunk@stusta.de>

drivers/md/md.c

@@ -1812,6 +1812,7 @@ static mdk_rdev_t *md_import_device(dev_t newdev, int super_format, int super_mi
 	kobject_init(&rdev->kobj);
 
 	rdev->desc_nr = -1;
+	rdev->saved_raid_disk = -1;
 	rdev->flags = 0;
 	rdev->data_offset = 0;
 	atomic_set(&rdev->nr_pending, 0);