selftests/bpf: Tweak cgroup kfunc test.

Adjust cgroup kfunc test to dereference RCU protected cgroup pointer
as PTR_TRUSTED and pass into KF_TRUSTED_ARGS kfunc.
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: David Vernet <void@manifault.com>
Link: https://lore.kernel.org/bpf/20230303041446.3630-6-alexei.starovoitov@gmail.com

tools/testing/selftests/bpf/progs/cgrp_kfunc_success.c

@@ -61,7 +61,7 @@ int BPF_PROG(test_cgrp_acquire_leave_in_map, struct cgroup *cgrp, const char *pa
 SEC("tp_btf/cgroup_mkdir")
 int BPF_PROG(test_cgrp_xchg_release, struct cgroup *cgrp, const char *path)
 {
-	struct cgroup *kptr;
+	struct cgroup *kptr, *cg;
 	struct __cgrps_kfunc_map_value *v;
 	long status;
 
@@ -80,6 +80,16 @@ int BPF_PROG(test_cgrp_xchg_release, struct cgroup *cgrp, const char *path)
 		return 0;
 	}
 
+	kptr = v->cgrp;
+	if (!kptr) {
+		err = 4;
+		return 0;
+	}
+
+	cg = bpf_cgroup_ancestor(kptr, 1);
+	if (cg)	/* verifier only check */
+		bpf_cgroup_release(cg);
+
 	kptr = bpf_kptr_xchg(&v->cgrp, NULL);
 	if (!kptr) {
 		err = 3;