net: iterate online nodes in skb_defer_free_flush()

skb_attempt_defer_free() only queues skbs on the current CPU's node
(numa_node_id() of a running CPU), which is always online, so the
flush loop never needs to visit nodes that are merely possible.

for_each_node() walks node_possible_map.  On machines where the
possible map is much larger than the online map -- e.g. a POWER10
LPAR with 32 possible but 1 online node -- the flush loop touches 31
cold, always-empty per-node lists on every softirq pass.

Use for_each_online_node() to iterate only node_online_map.  Nodes
that go offline are drained by dev_cpu_dead() ("net: flush
skb_defer_nodes in dev_cpu_dead()"), so the flush no longer needs to
visit offline nodes.

Loopback UDP throughput in a QEMU guest with 32 possible / 1 online
nodes (bench_udp, 8 senders, interleaved runs) improves by ~5%.

Fixes: 5628f3fe3b ("net: add NUMA awareness to skb_attempt_defer_free()")
Reported-by: kernel test robot <oliver.sang@intel.com>
Suggested-by: Adrian Tomasov <atomasov@redhat.com>
Closes: https://lore.kernel.org/oe-lkp/202512112119.5b9829a-lkp@intel.com
Signed-off-by: Kris Pan <kris.pan@intel.com>
Link: https://patch.msgid.link/20260928014708.3903028-1-kris.pan@intel.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/dev.c

@@ -6995,7 +6995,7 @@ static void skb_defer_free_flush(void)
 	struct skb_defer_node *sdn;
 	int node;
 
-	for_each_node(node) {
+	for_each_online_node(node) {
 		sdn = this_cpu_ptr(net_hotdata.skb_defer_nodes) + node;
 		__skb_defer_free_flush(sdn, 1);
 	}