md/raid5: allow removal for failed replacement devices.

Enhance raid5_remove_disk to be able to remove ->replacement
as well as ->rdev.
Reviewed-by: NDan Williams <dan.j.williams@intel.com>
Signed-off-by: NNeilBrown <neilb@suse.de>

drivers/md/raid5.c

@@ -5086,10 +5086,17 @@ static int raid5_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
 	struct r5conf *conf = mddev->private;
 	int err = 0;
 	int number = rdev->raid_disk;
+	struct md_rdev **rdevp;
 	struct disk_info *p = conf->disks + number;
 
 	print_raid5_conf(conf);
-	if (rdev == p->rdev) {
+	if (rdev == p->rdev)
+		rdevp = &p->rdev;
+	else if (rdev == p->replacement)
+		rdevp = &p->replacement;
+	else
+		return 0;
+
 	if (number >= conf->raid_disks &&
 	    conf->reshape_progress == MaxSector)
 		clear_bit(In_sync, &rdev->flags);
@@ -5109,13 +5116,12 @@ static int raid5_remove_disk(struct mddev *mddev, struct md_rdev *rdev)
 		err = -EBUSY;
 		goto abort;
 	}
-		p->rdev = NULL;
+	*rdevp = NULL;
 	synchronize_rcu();
 	if (atomic_read(&rdev->nr_pending)) {
 		/* lost the race, try later */
 		err = -EBUSY;
-			p->rdev = rdev;
-		}
+		*rdevp = rdev;
 	}
 abort: