MFV 286599: 5808 spa_check_logs is not necessary on readonly pools

Reviewed by: George Wilson <george@delphix.com>
Reviewed by: Paul Dagnelie <paul.dagnelie@delphix.com>
Reviewed by: Simon Klinkert <simon.klinkert@gmail.com>
Reviewed by: Will Andrews <will@freebsd.org>
Approved by: Gordon Ross <gwr@nexenta.com>
Author: Matthew Ahrens <mahrens@delphix.com>

illumos/illumos-gate@23367a2f2caec1ccb4d918bdd0f2fc2c9cadcd06