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: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Cc: Björn Töpel <bjorn.topel@intel.com>
Link: https://lore.kernel.org/linux-next/20201201190746.7d3357fb@canb.auug.org.au