[POWERPC] Make emergency stack safe for current_thread_info() use

The current_thread_info() macro, used by preempt_count(), assumes the
base address and size of the stack are THREAD_SIZE aligned.

The emergency stack currently isn't either of these things, which
could potentially cause problems anytime we're running on the
emergency stack.  That includes when we detect a bad kernel stack
pointer, and also during early_setup_secondary().
Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Paul Mackerras <paulus@samba.org>

arch/powerpc/kernel/setup_64.c

@@ -487,9 +487,12 @@ static void __init emergency_stack_init(void)
 	 */
 	limit = min(0x10000000UL, lmb.rmo_size);
 
-	for_each_possible_cpu(i)
-		paca[i].emergency_sp =
-		__va(lmb_alloc_base(HW_PAGE_SIZE, 128, limit)) + HW_PAGE_SIZE;
+	for_each_possible_cpu(i) {
+		unsigned long sp;
+		sp  = lmb_alloc_base(THREAD_SIZE, THREAD_SIZE, limit);
+		sp += THREAD_SIZE;
+		paca[i].emergency_sp = __va(sp);
+	}
 }
 
 /*