net, xdp, xsk: fix __sk_mark_napi_id_once napi_id error

Stephen reported the following build error for !CONFIG_NET_RX_BUSY_POLL
built kernels:

In file included from fs/select.c:32:
include/net/busy_poll.h: In function 'sk_mark_napi_id_once':
include/net/busy_poll.h:150:36: error: 'const struct sk_buff' has no member named 'napi_id'
150 | __sk_mark_napi_id_once_xdp(sk, skb->napi_id);
| ^~

Fix it by wrapping a CONFIG_NET_RX_BUSY_POLL around the helpers.

Fixes: b02e5a0ebb17 ("xsk: Propagate napi_id to XDP socket Rx path")
Reported-by: Stephen Rothwell <[e-mail]>
Signed-off-by: Stephen Rothwell <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Cc: Björn Töpel <[e-mail]>
Link: https://lore.kernel.org/linux-next/[e-mail]

include/net/busy_poll.h

···
 	sk_rx_queue_set(sk, skb);
 }
 
-static inline void __sk_mark_napi_id_once_xdp(struct sock *sk, unsigned int napi_id)
+static inline void __sk_mark_napi_id_once(struct sock *sk, unsigned int napi_id)
 {
 #ifdef CONFIG_NET_RX_BUSY_POLL
 	if (!READ_ONCE(sk->sk_napi_id))
···
 static inline void sk_mark_napi_id_once(struct sock *sk,
 					const struct sk_buff *skb)
 {
-	__sk_mark_napi_id_once_xdp(sk, skb->napi_id);
+#ifdef CONFIG_NET_RX_BUSY_POLL
+	__sk_mark_napi_id_once(sk, skb->napi_id);
+#endif
 }
 
 static inline void sk_mark_napi_id_once_xdp(struct sock *sk,
 					    const struct xdp_buff *xdp)
 {
-	__sk_mark_napi_id_once_xdp(sk, xdp->rxq->napi_id);
+#ifdef CONFIG_NET_RX_BUSY_POLL
+	__sk_mark_napi_id_once(sk, xdp->rxq->napi_id);
+#endif
 }
 
 #endif /* _LINUX_NET_BUSY_POLL_H */