bpf: Implement bpf_core_types_are_compat().

Adopt libbpf's bpf_core_types_are_compat() for kernel duty by adding
explicit recursion limit of 2 which is enough to handle 2 levels of
function prototypes.

Signed-off-by: Matteo Croce <mcroce@microsoft.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220204005519.60361-2-mcroce@linux.microsoft.com

include/linux/btf.h

@@ -327,6 +327,11 @@ static inline const struct btf_var_secinfo *btf_type_var_secinfo(
 	return (const struct btf_var_secinfo *)(t + 1);
 }
 
+static inline struct btf_param *btf_params(const struct btf_type *t)
+{
+	return (struct btf_param *)(t + 1);
+}
+
 #ifdef CONFIG_BPF_SYSCALL
 struct bpf_prog;