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

module/zfs/vdev.c

@@ -2520,7 +2520,8 @@ vdev_dtl_should_excise(vdev_t *vd)
 }
 
 /*
- * Reassess DTLs after a config change or scrub completion.
+ * Reassess DTLs after a config change or scrub completion. If txg == 0 no
+ * write operations will be issued to the pool.
  */
 void
 vdev_dtl_reassess(vdev_t *vd, uint64_t txg, uint64_t scrub_txg, int scrub_done)
@@ -2603,7 +2604,7 @@ vdev_dtl_reassess(vdev_t *vd, uint64_t txg, uint64_t scrub_txg, int scrub_done)
 		 * DTLs then reset its resilvering flag and dirty
 		 * the top level so that we persist the change.
 		 */
-		if (vd->vdev_resilver_txg != 0 &&
+		if (txg != 0 && vd->vdev_resilver_txg != 0 &&
 		    range_tree_is_empty(vd->vdev_dtl[DTL_MISSING]) &&
 		    range_tree_is_empty(vd->vdev_dtl[DTL_OUTAGE])) {
 			vd->vdev_resilver_txg = 0;