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>