libbpf: Add kptr type tag macros to bpf_helpers.h

Include convenience definitions:
__kptr:	Unreferenced kptr
__kptr_ref: Referenced kptr

Users can use them to tag the pointer type meant to be used with the new
support directly in the map value definition.

Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220424214901.2743946-11-memxor@gmail.com

tools/lib/bpf/bpf_helpers.h

@@ -149,6 +149,13 @@ enum libbpf_tristate {
 
 #define __kconfig __attribute__((section(".kconfig")))
 #define __ksym __attribute__((section(".ksyms")))
+#if __has_attribute(btf_type_tag)
+#define __kptr __attribute__((btf_type_tag("kptr")))
+#define __kptr_ref __attribute__((btf_type_tag("kptr_ref")))
+#else
+#define __kptr
+#define __kptr_ref
+#endif
 
 #ifndef ___bpf_concat
 #define ___bpf_concat(a, b) a ## b