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 <neilb@suse.de>