x86/boot/smp/32: Fix initial idle stack location on 32-bit kernels

On 32-bit kernels, the initial idle stack calculation doesn't take into
account the TOP_OF_KERNEL_STACK_PADDING, making the stack end address
inconsistent with other tasks on 32-bit.

Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Reviewed-by: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Nilay Vaish <nilayvaish@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/6cf569410bfa84cf923902fc4d628444cace94be.1474480779.git.jpoimboe@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/kernel/smpboot.c

@@ -964,9 +964,7 @@ static int do_boot_cpu(int apicid, int cpu, struct task_struct *idle)
 	int cpu0_nmi_registered = 0;
 	unsigned long timeout;
 
-	idle->thread.sp = (unsigned long) (((struct pt_regs *)
-			  (THREAD_SIZE +  task_stack_page(idle))) - 1);
-
+	idle->thread.sp = (unsigned long)task_pt_regs(idle);
 	early_gdt_descr.address = (unsigned long)get_cpu_gdt_table(cpu);
 	initial_code = (unsigned long)start_secondary;
 	initial_stack  = idle->thread.sp;