md/raid10: recognise replacements when assembling array.

If a Replacement is seen, file it as such.

If we see two replacements (or two normal devices) for the one slot,
abort.
Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/raid10.c

@@ -3201,6 +3201,16 @@ static int run(struct mddev *mddev)
 			continue;
 		disk = conf->mirrors + disk_idx;
 
+		if (test_bit(Replacement, &rdev->flags)) {
+			if (disk->replacement)
+				goto out_free_conf;
+			disk->replacement = rdev;
+		} else {
+			if (disk->rdev)
+				goto out_free_conf;
+			disk->rdev = rdev;
+		}
+
 		disk->rdev = rdev;
 		disk_stack_limits(mddev->gendisk, rdev->bdev,
 				  rdev->data_offset << 9);
@@ -3228,6 +3238,13 @@ static int run(struct mddev *mddev)
 
 		disk = conf->mirrors + i;
 
+		if (!disk->rdev && disk->replacement) {
+			/* The replacement is all we have - use it */
+			disk->rdev = disk->replacement;
+			disk->replacement = NULL;
+			clear_bit(Replacement, &disk->rdev->flags);
+		}
+
 		if (!disk->rdev ||
 		    !test_bit(In_sync, &disk->rdev->flags)) {
 			disk->head_position = 0;