powerpc: Avoid debug_smp_processor_id() check in SHARED_PROCESSOR

Use local_paca directly in macro SHARED_PROCESSOR, as all processors
have the same value for the field shared_proc, so we don't need care
racy here.

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