md/raid5: handle possible race as reshape completes.

It is possible (though unlikely) for a reshape to be
interrupted between the time that end_reshape is called
and the time when raid5_finish_reshape is called.

This can leave conf->reshape_progress set to MaxSector,
but mddev->reshape_position not.

This combination confused reshape_request() when ->reshape_backwards.
As conf->reshape_progress is so high, it seems the reshape hasn't
really begun.  But assuming MaxSector is a valid address only
leads to sorrow.

So ensure reshape_position and reshape_progress both agree,
and add an extra check in reshape_request() just in case they don't.
Signed-off-by: NeilBrown <neilb@suse.com>

drivers/md/raid5.c

@@ -5355,6 +5355,10 @@ static sector_t reshape_request(struct mddev *mddev, sector_t sector_nr, int *sk
 		    conf->reshape_progress < raid5_size(mddev, 0, 0)) {
 			sector_nr = raid5_size(mddev, 0, 0)
 				- conf->reshape_progress;
+		} else if (mddev->reshape_backwards &&
+			   conf->reshape_progress == MaxSector) {
+			/* shouldn't happen, but just in case, finish up.*/
+			sector_nr = MaxSector;
 		} else if (!mddev->reshape_backwards &&
 			   conf->reshape_progress > 0)
 			sector_nr = conf->reshape_progress;
@@ -7446,6 +7450,7 @@ static void end_reshape(struct r5conf *conf)
 			rdev->data_offset = rdev->new_data_offset;
 		smp_wmb();
 		conf->reshape_progress = MaxSector;
+		conf->mddev->reshape_position = MaxSector;
 		spin_unlock_irq(&conf->device_lock);
 		wake_up(&conf->wait_for_overlap);