powerpc: Initialise paca.data_offset with poison

It's possible for the cpu_possible_mask to change between the time we
initialise the pacas and the time we setup per_cpu areas.

Obviously impossible cpus shouldn't ever be running, but stranger things
have happened. So be paranoid and initialise data_offset with a poison
value in case we don't set it up later.

Based on a patch from Anton Blanchard.
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>

arch/powerpc/kernel/paca.c

@@ -142,6 +142,7 @@ void __init initialise_paca(struct paca_struct *new_paca, int cpu)
 	new_paca->hw_cpu_id = 0xffff;
 	new_paca->kexec_state = KEXEC_STATE_NONE;
 	new_paca->__current = &init_task;
+	new_paca->data_offset = 0xfeeeeeeeeeeeeeeeULL;
 #ifdef CONFIG_PPC_STD_MMU_64
 	new_paca->slb_shadow_ptr = &slb_shadow[cpu];
 #endif /* CONFIG_PPC_STD_MMU_64 */