md: allow a partially recovered device to be hot-added to an array.
When adding a new device into an array it is normally important to clear any stale data from ->recovery_offset else the new device may not be recovered properly. However when re-adding a device which is known to be nearly in-sync, this is not needed and can be detrimental. The (bitmap-based) resync will still happen, and further recovery is only needed from where-ever it was already up to. So if save_raid_disk is set, signifying a re-add, don't clear ->recovery_offset. Signed-off-by: NeilBrown <email@example.com>