powerpc: Use the new schedule_user API on userspace preemption

This patch corresponds to
[PATCH] x86: Use the new schedule_user API on userspace preemption
  commit 0430499c

Signed-off-by: Li Zhong <zhong@linux.vnet.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>