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: fd9c663 ("bpf: minimal support for programs hooked into netfilter framework")
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/oe-kbuild-all/202304220903.fRZTJtxe-lkp@intel.com/
Signed-off-by: Florian Westphal <fw@strlen.de>
Link: https://lore.kernel.org/r/20230422073544.17634-1-fw@strlen.de
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: Florian Westphal

include/linux/bpf_types.h

@@ -79,7 +79,7 @@ BPF_PROG_TYPE(BPF_PROG_TYPE_LSM, lsm,
 #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