bpf: Compile out btf_parse_module() if module BTF is not enabled

Make sure btf_parse_module() is compiled out if module BTFs are not enabled.

Fixes: 36e6844 ("bpf: Load and verify kernel module BTFs")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20201111040645.903494-1-andrii@kernel.org

kernel/bpf/btf.c

@@ -4478,6 +4478,8 @@ struct btf *btf_parse_vmlinux(void)
 	return ERR_PTR(err);
 }
 
+#ifdef CONFIG_DEBUG_INFO_BTF_MODULES
+
 static struct btf *btf_parse_module(const char *module_name, const void *data, unsigned int data_size)
 {
 	struct btf_verifier_env *env = NULL;
@@ -4547,6 +4549,8 @@ static struct btf *btf_parse_module(const char *module_name, const void *data, u
 	return ERR_PTR(err);
 }
 
+#endif /* CONFIG_DEBUG_INFO_BTF_MODULES */
+
 struct btf *bpf_prog_get_target_btf(const struct bpf_prog *prog)
 {
 	struct bpf_prog *tgt_prog = prog->aux->dst_prog;