bpf: Add stub for btf_struct_access()

Add corresponding unimplemented stub for when CONFIG_BPF_SYSCALL=n
Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/4021398e884433b1fef57a4d28361bb9fcf1bd05.1662568410.git.dxu@dxuuu.xyzSigned-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/bpf.h

@@ -2211,6 +2211,15 @@ static inline struct bpf_prog *bpf_prog_by_id(u32 id)
 	return ERR_PTR(-ENOTSUPP);
 }
 
+static inline int btf_struct_access(struct bpf_verifier_log *log,
+				    const struct btf *btf,
+				    const struct btf_type *t, int off, int size,
+				    enum bpf_access_type atype,
+				    u32 *next_btf_id, enum bpf_type_flag *flag)
+{
+	return -EACCES;
+}
+
 static inline const struct bpf_func_proto *
 bpf_base_func_proto(enum bpf_func_id func_id)
 {