Fix dirtying vdev config on with RO spa

This patch simply corrects an issue where vdev_dtl_reassess()
could attempt to dirty the vdev config even when the spa was
not elligable for writing.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #8085