uma: make UMA_RECLAIM_DRAIN_CPU drain SMR free buckets

This very aggressive drain should probably be encoded with
a different request.  Now it is a PoC that allows to drain
all pcb zones on VNET jail destroy.

sys/vm/uma_core.c

@@ -1448,22 +1448,20 @@ cache_drain_safe_cpu(uma_zone_t zone, void *unused)
 	cache = &zone->uz_cpu[curcpu];
 	domain = PCPU_GET(domain);
 	b1 = cache_bucket_unload_alloc(cache);
-
-	/*
-	 * Don't flush SMR zone buckets.  This leaves the zone without a
-	 * bucket and forces every free to synchronize().
-	 */
-	if ((zone->uz_flags & UMA_ZONE_SMR) == 0) {
-		b2 = cache_bucket_unload_free(cache);
-		b3 = cache_bucket_unload_cross(cache);
-	}
+	b2 = cache_bucket_unload_free(cache);
+	b3 = cache_bucket_unload_cross(cache);
 	critical_exit();
 
 	if (b1 != NULL)
 		zone_free_bucket(zone, b1, NULL, domain, false);
-	if (b2 != NULL)
+	if (b2 != NULL) {
+		if ((zone->uz_flags & UMA_ZONE_SMR) != 0)
+			b2->ub_seq = smr_advance(zone->uz_smr);
 		zone_free_bucket(zone, b2, NULL, domain, false);
+	}
 	if (b3 != NULL) {
+		if ((zone->uz_flags & UMA_ZONE_SMR) != 0)
+			b3->ub_seq = smr_advance(zone->uz_smr);
 		/* Adjust the domain so it goes to zone_free_cross. */
 		domain = (domain + 1) % vm_ndomains;
 		zone_free_bucket(zone, b3, NULL, domain, false);