sched: Trigger warning if ->migration_disabled counter underflows.

If migrate_enable() is used more often than its counter part then it
remains undetected and rq::nr_pinned will underflow, too.

Add a warning if migrate_enable() is attempted if without a matching a
migrate_disable().
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20211129174654.668506-2-bigeasy@linutronix.de

kernel/sched/core.c

@@ -2173,6 +2173,9 @@ void migrate_enable(void)
 		return;
 	}
 
+	if (WARN_ON_ONCE(!p->migration_disabled))
+		return;
+
 	/*
 	 * Ensure stop_task runs either before or after this, and that
 	 * __set_cpus_allowed_ptr(SCA_MIGRATE_ENABLE) doesn't schedule().