sched_ext: selftests/dsp_local_on: Fix selftest on UP systems

In UP systems p->migration_disabled is not available. Fix this by using
the portable helper is_migration_disabled(p).

Fixes: e9fe182 ("sched_ext: selftests/dsp_local_on: Fix sporadic failures")
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

Author: arighi

tools/testing/selftests/sched_ext/dsp_local_on.bpf.c

@@ -43,7 +43,7 @@ void BPF_STRUCT_OPS(dsp_local_on_dispatch, s32 cpu, struct task_struct *prev)
 	if (!p)
 		return;
 
-	if (p->nr_cpus_allowed == nr_cpus && !p->migration_disabled)
+	if (p->nr_cpus_allowed == nr_cpus && !is_migration_disabled(p))
 		target = bpf_get_prandom_u32() % nr_cpus;
 	else
 		target = scx_bpf_task_cpu(p);