powerpc: Use call_rcu_sched() for pagetables

PowerPC relies on IRQ-disable to guard against RCU quiecent states,
use the appropriate RCU call version.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>