powerpc: Access local paca after hard irq disabled

In hard_irq_disable(), we accessed the PACA before we hard disabled
the interrupts, potentially causing a warning as get_paca() will
us debug_smp_processor_id().

Move that to after the disabling, and also use local_paca directly
rather than get_paca() to avoid several redundant and useless checks.

Signed-off-by: Tiejun Chen <tiejun.chen@windriver.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>