md: fix regression with re-adding devices to arrays with no metadata

Commit 1a855a06 (2.6.37-rc4) fixed a problem where devices were
re-added when they shouldn't be but caused a regression in a less
common case that means sometimes devices cannot be re-added when they
should be.

In particular, when re-adding a device to an array without metadata
we should always access the device, but after the above commit we
didn't.

This patch sets the In_sync flag in that case so that the re-add
succeeds.

This patch is suitable for any -stable kernel to which 1a855a06 was
applied.

Cc: stable@kernel.org
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/md.c

@@ -5159,9 +5159,10 @@ static int add_new_disk(mddev_t * mddev, mdu_disk_info_t *info)
 		/* set saved_raid_disk if appropriate */
 		if (!mddev->persistent) {
 			if (info->state & (1<<MD_DISK_SYNC)  &&
-			    info->raid_disk < mddev->raid_disks)
+			    info->raid_disk < mddev->raid_disks) {
 				rdev->raid_disk = info->raid_disk;
-			else
+				set_bit(In_sync, &rdev->flags);
+			} else
 				rdev->raid_disk = -1;
 		} else
 			super_types[mddev->major_version].