TEST: use __bpf_md_ptr() in struct prog_test_ref_kfunc

Try same as b7df9ad ("bpf: fix pointer offsets in context for 32 bit")

Fix for following but doesnt work...:

 - torvalds#128/p calls: invalid kfunc call: referenced arg needs refcounted PTR_TO_BTF_ID FAIL

 - torvalds#481/p map_kptr: unref: inherit PTR_UNTRUSTED on struct walk FAIL

Author: guidosarducci

net/bpf/test_run.c

@@ -611,7 +611,7 @@ struct prog_test_ref_kfunc {
 	int a;
 	int b;
 	struct prog_test_member memb;
-	struct prog_test_ref_kfunc *next;
+	__bpf_md_ptr(struct prog_test_ref_kfunc, *next);
 	refcount_t cnt;
 };
 

tools/testing/selftests/bpf/test_kmods/bpf_testmod_kfunc.h

@@ -21,7 +21,7 @@ struct prog_test_ref_kfunc {
 	int a;
 	int b;
 	struct prog_test_member memb;
-	struct prog_test_ref_kfunc *next;
+	__bpf_md_ptr(struct prog_test_ref_kfunc, *next);
 	refcount_t cnt;
 };
 #endif