md: Remove recent change which allows devices to skip recovery.

commit 7ceb17e87bde79d285a8b988cfed9eaeebe60b86
    md: Allow devices to be re-added to a read-only array.

allowed a bit more than just that.  It also allows devices to be added
to a read-write array and to end up skipping recovery.

This patch removes the offending piece of code pending a rewrite for a
subsequent release.

More specifically:
 If the array has a bitmap, then the device will still need a bitmap
 based resync ('saved_raid_disk' is set under different conditions
 is a bitmap is present).
 If the array doesn't have a bitmap, then this is correct as long as
 nothing has been written to the array since the metadata was checked
 by ->validate_super.  However there is no locking to ensure that there
 was no write.

Bug was introduced in 3.10 and causes data corruption so
patch is suitable for 3.10-stable.

Cc: stable@vger.kernel.org (3.10)
Reported-by: Joe Lawrence <joe.lawrence@stratus.com>
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/md.c

@@ -7716,20 +7716,6 @@ static int remove_and_add_spares(struct mddev *mddev,
 			continue;
 
 		rdev->recovery_offset = 0;
-		if (rdev->saved_raid_disk >= 0 && mddev->in_sync) {
-			spin_lock_irq(&mddev->write_lock);
-			if (mddev->in_sync)
-				/* OK, this device, which is in_sync,
-				 * will definitely be noticed before
-				 * the next write, so recovery isn't
-				 * needed.
-				 */
-				rdev->recovery_offset = mddev->recovery_cp;
-			spin_unlock_irq(&mddev->write_lock);
-		}
-		if (mddev->ro && rdev->recovery_offset != MaxSector)
-			/* not safe to add this disk now */
-			continue;
 		if (mddev->pers->
 		    hot_add_disk(mddev, rdev) == 0) {
 			if (sysfs_link_rdev(mddev, rdev))