x86: remove magic number from ACPI sleep stack buffer

x86_64 SMP suspend to RAM uses a 10k temporary stack for saving the
kernel state, but only 4k of it is used. Shrink it to 4k.
Signed-off-by: Matt Mackall <mpm@selenic.com>
Acked-by: Pavel Machek <pavel@suse.cz>
Signed-off-by: Len Brown <len.brown@intel.com>

arch/x86/kernel/acpi/sleep.c

@@ -97,7 +97,7 @@ int acpi_save_state_mem(void)
 #else /* CONFIG_64BIT */
 	header->trampoline_segment = setup_trampoline() >> 4;
 #ifdef CONFIG_SMP
-	stack_start.sp = temp_stack + 4096;
+	stack_start.sp = temp_stack + sizeof(temp_stack);
 #endif
 	initial_code = (unsigned long)wakeup_long64;
 	saved_magic = 0x123456789abcdef0;