sched_ext: selftests/dsp_local_on: Fix selftest on UP systems

commit 3c7d51b0 upstream.

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

Fixes: e9fe1827 ("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>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>