bpf, netns: Fix build without CONFIG_INET

When CONFIG_NET is set but CONFIG_INET isn't, build fails with:

  ld: kernel/bpf/net_namespace.o: in function `netns_bpf_attach_type_unneed':
  kernel/bpf/net_namespace.c:32: undefined reference to `bpf_sk_lookup_enabled'
  ld: kernel/bpf/net_namespace.o: in function `netns_bpf_attach_type_need':
  kernel/bpf/net_namespace.c:43: undefined reference to `bpf_sk_lookup_enabled'

This is because without CONFIG_INET bpf_sk_lookup_enabled symbol is not
available. Wrap references to bpf_sk_lookup_enabled with preprocessor
conditionals.

Fixes: 1559b4aa1d ("inet: Run SK_LOOKUP BPF program on socket lookup")
Reported-by: Randy Dunlap <rdunlap@infradead.org>
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Link: https://lore.kernel.org/bpf/20200721100716.720477-1-jakub@cloudflare.com

kernel/bpf/net_namespace.c

@@ -28,9 +28,11 @@ DEFINE_MUTEX(netns_bpf_mutex);
 static void netns_bpf_attach_type_unneed(enum netns_bpf_attach_type type)
 {
 	switch (type) {
+#ifdef CONFIG_INET
 	case NETNS_BPF_SK_LOOKUP:
 		static_branch_dec(&bpf_sk_lookup_enabled);
 		break;
+#endif
 	default:
 		break;
 	}
@@ -39,9 +41,11 @@ static void netns_bpf_attach_type_unneed(enum netns_bpf_attach_type type)
 static void netns_bpf_attach_type_need(enum netns_bpf_attach_type type)
 {
 	switch (type) {
+#ifdef CONFIG_INET
 	case NETNS_BPF_SK_LOOKUP:
 		static_branch_inc(&bpf_sk_lookup_enabled);
 		break;
+#endif
 	default:
 		break;
 	}