x86/idt: Load idt early in start_secondary

On a secondary, idt is first loaded in cpu_init() with load_current_idt(),
i.e. no exceptions can be handled before that point.

The conversion of WARN() to use UD requires the IDT being loaded earlier as
any warning between start_secondary() and load_curren_idt() in cpu_init()
will result in an unhandled @UD exception and therefore fail the bringup of
the CPU.

Install the IDT handlers right in start_secondary() before calling cpu_init().

[ tglx: Massaged changelog ]

Fixes: 9a93848f ("x86/debug: Implement __WARN() using UD0")
Signed-off-by: Chunyu Hu <chuhu@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Cc: peterz@infradead.org
Cc: bp@alien8.de
Cc: rostedt@goodmis.org
Cc: luto@kernel.org
Link: https://lkml.kernel.org/r/1511792499-4073-1-git-send-email-chuhu@redhat.com

arch/x86/kernel/smpboot.c

@@ -237,7 +237,7 @@ static void notrace start_secondary(void *unused)
 	load_cr3(swapper_pg_dir);
 	__flush_tlb_all();
 #endif
-
+	load_current_idt();
 	cpu_init();
 	x86_cpuinit.early_percpu_clock_init();
 	preempt_disable();