[ARM] Stack starts at THREAD_START_SP offset, not THREAD_SIZE-8

Use the correct constants.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
diff --git a/arch/arm/kernel/smp.c b/arch/arm/kernel/smp.c
index b208573..8261649 100644
--- a/arch/arm/kernel/smp.c
+++ b/arch/arm/kernel/smp.c
@@ -110,7 +110,7 @@
 	 * We need to tell the secondary core where to find
 	 * its stack and the page tables.
 	 */
-	secondary_data.stack = (void *)idle->thread_info + THREAD_SIZE - 8;
+	secondary_data.stack = (void *)idle->thread_info + THREAD_START_SP;
 	secondary_data.pgdir = virt_to_phys(pgd);
 	wmb();