powerpc: Rename THREAD_INFO to TASK_STACK

This patch renames THREAD_INFO to TASK_STACK, because it is in fact
the offset of the pointer to the stack in task_struct so this pointer
will not be impacted by the move of THREAD_INFO.

Also make it available on 64-bit, as we'll need it there when we
activate THREAD_INFO_IN_TASK.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Nicholas Piggin <npiggin@gmail.com>
[mpe: Make available on 64-bit]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/kernel/entry_32.S

@@ -1165,7 +1165,7 @@ ret_from_debug_exc:
 	mfspr	r9,SPRN_SPRG_THREAD
 	lwz	r10,SAVED_KSP_LIMIT(r1)
 	stw	r10,KSP_LIMIT(r9)
-	lwz	r9,THREAD_INFO-THREAD(r9)
+	lwz	r9,TASK_STACK-THREAD(r9)
 	CURRENT_THREAD_INFO(r10, r1)
 	lwz	r10,TI_PREEMPT(r10)
 	stw	r10,TI_PREEMPT(r9)