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@973c78e94bf9634782164382c9e291bf81161fa5

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