rationalise return value for ->hot_add_disk method.

For all array types but linear, ->hot_add_disk returns 1 on
success, 0 on failure.
For linear, it returns 0 on success and -errno on failure.

This doesn't cause a functional problem because the ->hot_add_disk
function of linear is used quite differently to the others.
However it is confusing.

So convert all to return 0 for success or -errno on failure
and fix call sites to match.

Signed-off-by: Neil Brown <neilb@suse.de>
diff --git a/drivers/md/multipath.c b/drivers/md/multipath.c
index 4a1d714..541cbe3 100644
--- a/drivers/md/multipath.c
+++ b/drivers/md/multipath.c
@@ -281,7 +281,7 @@
 {
 	multipath_conf_t *conf = mddev->private;
 	struct request_queue *q;
-	int found = 0;
+	int err = -EEXIST;
 	int path;
 	struct multipath_info *p;
 	int first = 0;
@@ -312,11 +312,13 @@
 			rdev->raid_disk = path;
 			set_bit(In_sync, &rdev->flags);
 			rcu_assign_pointer(p->rdev, rdev);
-			found = 1;
+			err = 0;
+			break;
 		}
 
 	print_multipath_conf(conf);
-	return found;
+
+	return err;
 }
 
 static int multipath_remove_disk(mddev_t *mddev, int number)