selftests/bpf: Fix a selftest compilation error with CONFIG_SMP=n

Kernel test robot reported bpf selftest build failure when CONFIG_SMP
is not set. The error message looks below:

>> progs/rcu_read_lock.c:256:34: error: no member named 'last_wakee' in 'struct task_struct'
last_wakee = task->real_parent->last_wakee;
~~~~~~~~~~~~~~~~~ ^
1 error generated.

When CONFIG_SMP is not set, the field 'last_wakee' is not available in struct
'task_struct'. Hence the above compilation failure. To fix the issue, let us
choose another field 'group_leader' which is available regardless of
CONFIG_SMP set or not.

Fixes: fe147956fca4 ("bpf/selftests: Add selftests for new task kfuncs")
Fixes: 48671232fcb8 ("selftests/bpf: Add tests for bpf_rcu_read_lock()")
Reported-by: kernel test robot <[e-mail]>
Signed-off-by: David Vernet <[e-mail]>
Signed-off-by: Yonghong Song <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Signed-off-by: Daniel Borkmann <[e-mail]>
Acked-by: David Vernet <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

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

···
 SEC("?fentry.s/" SYS_PREFIX "sys_getpgid")
 int task_untrusted_non_rcuptr(void *ctx)
 {
-	struct task_struct *task, *last_wakee;
+	struct task_struct *task, *group_leader;
 
 	task = bpf_get_current_task_btf();
 	bpf_rcu_read_lock();
-	/* the pointer last_wakee marked as untrusted */
-	last_wakee = task->real_parent->last_wakee;
-	(void)bpf_task_storage_get(&map_a, last_wakee, 0, 0);
+	/* the pointer group_leader marked as untrusted */
+	group_leader = task->real_parent->group_leader;
+	(void)bpf_task_storage_get(&map_a, group_leader, 0, 0);
 	bpf_rcu_read_unlock();
 	return 0;
 }

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

···
 	struct task_struct *acquired;
 
 	/* Can't invoke bpf_task_acquire() on a trusted pointer obtained from walking a struct. */
-	acquired = bpf_task_acquire(task->last_wakee);
+	acquired = bpf_task_acquire(task->group_leader);
 	bpf_task_release(acquired);
 
 	return 0;