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>

drivers/md/md.c

@@ -7736,7 +7736,8 @@ static int remove_and_add_spares(struct mddev *mddev,
 		       !test_bit(Bitmap_sync, &rdev->flags)))
 			continue;
 
-		rdev->recovery_offset = 0;
+		if (rdev->saved_raid_disk < 0)
+			rdev->recovery_offset = 0;
 		if (mddev->pers->
 		    hot_add_disk(mddev, rdev) == 0) {
 			if (sysfs_link_rdev(mddev, rdev))