Illumos #4121 vdev_label_init read only

4121 vdev_label_init should treat request as succeeded when pool
     is read only
Reviewed by: Christopher Siden <christopher.siden@delphix.com>
Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Reviewed by: Saso Kiselkov <skiselkov.ml@gmail.com>
Approved by: Richard Lowe <richlowe@richlowe.net>

References:
  https://www.illumos.org/issues/4121
  illumos/illumos-gate@973c78e

Ported-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1863

Author: grwilson

module/zfs/vdev_label.c

@@ -644,7 +644,7 @@ vdev_label_init(vdev_t *vd, uint64_t crtxg, vdev_labeltype_t reason)
 	/* Track the creation time for this vdev */
 	vd->vdev_crtxg = crtxg;
 
-	if (!vd->vdev_ops->vdev_op_leaf)
+	if (!vd->vdev_ops->vdev_op_leaf || !spa_writeable(spa))
 		return (0);
 
 	/*