bpf: Fix build without CONFIG_NET when using BPF XDP link

Entire net/core subsystem is not built without CONFIG_NET. linux/netdevice.h
just assumes that it's always there, so the easiest way to fix this is to
conditionally compile out bpf_xdp_link_attach() use in bpf/syscall.c.

Fixes: aa8d3a7 ("bpf, xdp: Add bpf_link-based XDP attachment API")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200728190527.110830-1-andriin@fb.com

kernel/bpf/syscall.c

@@ -3923,9 +3923,11 @@ static int link_create(union bpf_attr *attr)
  case BPF_PROG_TYPE_SK_LOOKUP:
  ret = netns_bpf_link_create(attr, prog);
  break;
+#ifdef CONFIG_NET
  case BPF_PROG_TYPE_XDP:
  ret = bpf_xdp_link_attach(attr, prog);
  break;
+#endif
  default:
  ret = -EINVAL;
  }