selftests/bpf: Add __contains macro to bpf_experimental.h

Add user facing __contains macro which provides a convenient wrapper
over the verbose kernel specific BTF declaration tag required to
annotate BPF list head structs in user types.

Acked-by: Dave Marchevsky <[e-mail]>
Signed-off-by: Kumar Kartikeya Dwivedi <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Alexei Starovoitov <[e-mail]>

tools/testing/selftests/bpf/bpf_experimental.h

···
 #include <bpf/bpf_helpers.h>
 #include <bpf/bpf_core_read.h>
 
+#define __contains(name, node) __attribute__((btf_decl_tag("contains:" #name ":" #node)))
+
 /* Description
  *	Allocates an object of the type represented by 'local_type_id' in
  *	program BTF. User may use the bpf_core_type_id_local macro to pass the