netpoll: Fix race condition in netpoll_owner_active

KCSAN detected a race condition in netpoll:

	BUG: KCSAN: data-race in net_rx_action / netpoll_send_skb
	write (marked) to 0xffff8881164168b0 of 4 bytes by interrupt on cpu 10:
	net_rx_action (./include/linux/netpoll.h:90 net/core/dev.c:6712 net/core/dev.c:6822)
<snip>
	read to 0xffff8881164168b0 of 4 bytes by task 1 on cpu 2:
	netpoll_send_skb (net/core/netpoll.c:319 net/core/netpoll.c:345 net/core/netpoll.c:393)
	netpoll_send_udp (net/core/netpoll.c:?)
<snip>
	value changed: 0x0000000a -> 0xffffffff

This happens because netpoll_owner_active() needs to check if the
current CPU is the owner of the lock, touching napi->poll_owner
non atomically. The ->poll_owner field contains the current CPU holding
the lock.

Use an atomic read to check if the poll owner is the current CPU.
Signed-off-by: Breno Leitao <leitao@debian.org>
Link: https://lore.kernel.org/r/20240429100437.3487432-1-leitao@debian.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

net/core/netpoll.c

@@ -316,7 +316,7 @@ static int netpoll_owner_active(struct net_device *dev)
 	struct napi_struct *napi;
 
 	list_for_each_entry_rcu(napi, &dev->napi_list, dev_list) {
-		if (napi->poll_owner == smp_processor_id())
+		if (READ_ONCE(napi->poll_owner) == smp_processor_id())
 			return 1;
 	}
 	return 0;