bpf: fix link failure with NETFILTER=y INET=n

Explicitly check if NETFILTER_BPF_LINK is enabled, else configs
that have NETFILTER=y but CONFIG_INET=n fail to link:

> kernel/bpf/syscall.o: undefined reference to `netfilter_prog_ops'
> kernel/bpf/verifier.o: undefined reference to `netfilter_verifier_ops'

Fixes: fd9c663b9ad6 ("bpf: minimal support for programs hooked into netfilter framework")
Reported-by: kernel test robot <[e-mail]>
Link: https://lore.kernel.org/oe-kbuild-all/[e-mail]/
Signed-off-by: Florian Westphal <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

include/linux/bpf_types.h

···
 #endif
 BPF_PROG_TYPE(BPF_PROG_TYPE_SYSCALL, bpf_syscall,
 	      void *, void *)
-#ifdef CONFIG_NETFILTER
+#ifdef CONFIG_NETFILTER_BPF_LINK
 BPF_PROG_TYPE(BPF_PROG_TYPE_NETFILTER, netfilter,
 	      struct bpf_nf_ctx, struct bpf_nf_ctx)
 #endif