FreeBSD rS271776 - Persist vdev_resilver_txg changes

Persist vdev_resilver_txg changes to avoid panic caused by validation
vs a vdev_resilver_txg value from a previous resilver.

Authored-by: smh <smh@FreeBSD.org>
Ported-by: Chris Dunlop <chris@onthe.net.au>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

OpenZFS-issue: https://www.illumos.org/issues/5154
FreeBSD-issue: https://reviews.freebsd.org/rS271776
FreeBSD-commit: freebsd/freebsd-src@c3c60bf
Closes openzfs#4790

module/zfs/vdev.c

@@ -1894,12 +1894,15 @@ vdev_dtl_reassess(vdev_t *vd, uint64_t txg, uint64_t scrub_txg, int scrub_done)
 
 		/*
 		 * If the vdev was resilvering and no longer has any
-		 * DTLs then reset its resilvering flag.
+		 * DTLs then reset its resilvering flag and dirty
+		 * the top level so that we persist the change.
 		 */
 		if (vd->vdev_resilver_txg != 0 &&
 		    range_tree_space(vd->vdev_dtl[DTL_MISSING]) == 0 &&
-		    range_tree_space(vd->vdev_dtl[DTL_OUTAGE]) == 0)
+		    range_tree_space(vd->vdev_dtl[DTL_OUTAGE]) == 0) {
 			vd->vdev_resilver_txg = 0;
+			vdev_config_dirty(vd->vdev_top);
+		}
 
 		mutex_exit(&vd->vdev_dtl_lock);