MFV r329776: 8477 Assertion failed in vdev_state_dirty(): spa_writeable(spa)

MFV r329776: 8477 Assertion failed in vdev_state_dirty(): spa_writeable(spa)

illumos/illumos-gate@f4c1745bd6c9829a05ecec15759ede7757100ab5

Illumos 4080 allows "zpool clear" to work on readonly pools: i don't think
this is the intended behaviour, we shouldn't be allowed to clear readonly
pools. Probably.

A fix is already in the ZFS on Linux repository to addess this issue:
https://github.com/zfsonlinux/zfs/commit/92e43c17188d47f47b69318e4884096dec380e36

Reviewed by: Matthew Ahrens <mahrens@delphix.com>
Approved by: Dan McDonald <danmcd@joyent.com>
Author: loli10K <ezomori.nozomu@gmail.com>