Illumos 5808 - spa_check_logs is not necessary on readonly pools

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>

References:

https://www.illumos.org/issues/5808
https://github.com/illumos/illumos-gate/commit/23367a2f

Ported-by: DHE <git@dehacked.net>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #3369