Moving the metaslab_group_set_rotor_category() up.

ztest did an (unreproducible) failure with stack trace of
spa_tryimport() -> ... vdev_load() -> ... vdev_metaslab_init() ->
metaslab_init() -> ... metaslab_class_space_update() that failed its
ASSERT.  Inspection showed that vdev_metaslab_init() would soon call
metaslab_group_activate(), i.e. we need to assign mg_nrot.
(Hopefully, vdev_open was called earlier...?)

module/zfs/vdev.c

@@ -1231,6 +1231,20 @@ vdev_open(vdev_t *vd)
 
 	error = vd->vdev_ops->vdev_op_open(vd, &osize, &max_osize, &ashift);
 
+	/*
+	 * Somewhere after vd->vdev_ops->vdev_op_open() (that calls
+	 * vdev_open_children() and thus updates vd->vdev_nonrot) and
+	 * before metaslab_class_space_update() gets called, we need
+	 * to assign the rotor category of the metaslab group.
+	 *
+	 * At this point, the metaslab group has already been created.
+	 *
+	 * Do the handling here, before any return in case error has
+	 * been set.
+	 */
+	if (vd->vdev_mg)
+		metaslab_group_set_rotor_category(vd->vdev_mg);
+
 	/*
 	 * Reset the vdev_reopening flag so that we actually close
 	 * the vdev on error.
@@ -1388,18 +1402,6 @@ vdev_open(vdev_t *vd)
 	    vdev_resilver_needed(vd, NULL, NULL))
 		spa_async_request(spa, SPA_ASYNC_RESILVER);
 
-	/*
-	 * Somewhere after vd->vdev_ops->vdev_op_open() (that calls
-	 * vdev_open_children() and thus updates vd->vdev_nonrot) and
-	 * before metaslab_class_space_update() gets called, we need
-	 * to assign the rotor category of the metaslab group.
-	 *
-	 * At this point, the metaslab group has already been created,
-	 * so go for it.
-	 */
-	if (vd->vdev_mg)
-		metaslab_group_set_rotor_category(vd->vdev_mg);
-
 	return (0);
 }