mm: workingset: replace IRQ-off check with a lockdep assert.

Commit

  68d48e6a ("mm: workingset: add vmstat counter for shadow nodes")

introduced an IRQ-off check to ensure that a lock is held which also
disabled interrupts. This does not work the same way on -RT because none
of the locks, that are held, disable interrupts.
Replace this check with a lockdep assert which ensures that the lock is
held.

Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190211113829.sqf6bdi4c4cdd3rp@linutronix.de