lockdep/selftests: Restore migrate_disable() state on PREEMPT_RT

The lockdep selftests deliberately run unbalanced locking patterns.
dotest() restores the task state they leave behind before running the
next testcase.

On PREEMPT_RT, spin_lock() uses migrate_disable() instead of disabling
preemption. dotest() cleans up the resulting migration-disabled state, but
that cleanup is still guarded by CONFIG_SMP.

That used to match the scheduler data model, where migration_disabled was
also CONFIG_SMP-only. The commit referenced below made SMP scheduler state
unconditional, so CONFIG_SMP=n PREEMPT_RT kernels with
CONFIG_DEBUG_LOCKING_API_SELFTESTS=y report success from the selftests and
then trip over stale current->migration_disabled state:

  releasing a pinned lock
  bad: scheduling from the idle thread!
  Kernel panic - not syncing: Fatal exception

Save and restore current->migration_disabled for every PREEMPT_RT build.

Fixes: cac5cefbad ("sched/smp: Make SMP unconditional")
Assisted-by: Codex:gpt-5
Signed-off-by: Karl Mehltretter <kmehltretter@gmail.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://patch.msgid.link/20260523185123.17482-2-kmehltretter@gmail.com

lib/locking-selftest.c

@@ -1431,9 +1431,7 @@ static void dotest(void (*testcase_fn)(void), int expected, int lockclass_mask)
 {
 	int saved_preempt_count = preempt_count();
 #ifdef CONFIG_PREEMPT_RT
-#ifdef CONFIG_SMP
 	int saved_mgd_count = current->migration_disabled;
-#endif
 	int saved_rcu_count = current->rcu_read_lock_nesting;
 #endif
 
@@ -1471,10 +1469,8 @@ static void dotest(void (*testcase_fn)(void), int expected, int lockclass_mask)
 	preempt_count_set(saved_preempt_count);
 
 #ifdef CONFIG_PREEMPT_RT
-#ifdef CONFIG_SMP
 	while (current->migration_disabled > saved_mgd_count)
 		migrate_enable();
-#endif
 
 	while (current->rcu_read_lock_nesting > saved_rcu_count)
 		rcu_read_unlock();