srcu: replace local_irqsave() with a locallock

There are two instances which disable interrupts in order to become a
stable this_cpu_ptr() pointer. The restore part is coupled with
spin_unlock_irqrestore() which does not work on RT.
Replace the local_irq_save() call with the appropriate local_lock()
version of it.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>