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>