xsk: Bring back busy polling support in XDP_COPY

Commit 5ef44b3 ("xsk: Bring back busy polling support") fixed the
busy polling support in xsk for XDP_ZEROCOPY after it was broken in
commit 86e25f4 ("net: napi: Add napi_config"). The busy polling
support with XDP_COPY remained broken since the napi_id setup in
xsk_rcv_check was removed.

Bring back the setup of napi_id for XDP_COPY so socket level SO_BUSYPOLL
can be used to poll the underlying napi.

Do the setup of napi_id for XDP_COPY in xsk_bind, as it is done
currently for XDP_ZEROCOPY. The setup of napi_id for XDP_COPY in
xsk_bind is safe because xsk_rcv_check checks that the rx queue at which
the packet arrives is equal to the queue_id that was supplied in bind.
This is done for both XDP_COPY and XDP_ZEROCOPY mode.

Tested using AF_XDP support in virtio-net by running the xsk_rr AF_XDP
benchmarking tool shared here:
https://lore.kernel.org/all/20250320163523.3501305-1-skhawaja@google.com/T/

Enabled socket busy polling using following commands in qemu,

```
sudo ethtool -L eth0 combined 1
echo 400 | sudo tee /proc/sys/net/core/busy_read
echo 100 | sudo tee /sys/class/net/eth0/napi_defer_hard_irqs
echo 15000   | sudo tee /sys/class/net/eth0/gro_flush_timeout
```

Fixes: 5ef44b3 ("xsk: Bring back busy polling support")
Signed-off-by: Samiullah Khawaja <skhawaja@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Signed-off-by: David S. Miller <davem@davemloft.net>

Author: samikhawaja

net/xdp/xsk.c

@@ -1304,7 +1304,7 @@ static int xsk_bind(struct socket *sock, struct sockaddr *addr, int addr_len)
 	xs->queue_id = qid;
 	xp_add_xsk(xs->pool, xs);
 
-	if (xs->zc && qid < dev->real_num_rx_queues) {
+	if (qid < dev->real_num_rx_queues) {
 		struct netdev_rx_queue *rxq;
 
 		rxq = __netif_get_rx_queue(dev, qid);