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: b02e5a0e ("xsk: Propagate napi_id to XDP socket Rx path")
Reported-by: NStephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NStephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>
Cc: Björn Töpel <bjorn.topel@intel.com>
Link: https://lore.kernel.org/linux-next/20201201190746.7d3357fb@canb.auug.org.au

include/net/busy_poll.h

@@ -135,7 +135,7 @@ static inline void sk_mark_napi_id(struct sock *sk, const struct sk_buff *skb)
 	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))
@@ -147,13 +147,17 @@ static inline void __sk_mark_napi_id_once_xdp(struct sock *sk, unsigned int napi
 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 */