x86: fix boot crash in NMI watchdog with CONFIG_CPUMASK_OFFSTACK=y and flat APIC

fcef8576 converted backtrace_mask to a
cpumask_var_t, and assumed check_nmi_watchdog was called before
nmi_watchdog_tick was ever called.  Steven's oops shows I was wrong.

This is something of a bandaid: I'm not sure we *should* be calling
nmi_watchdog_tick before check_nmi_watchdog.  Note that gcc eliminates
this test for the CONFIG_CPUMASK_OFFSTACK=n case.

[ Impact: fix boot crash in rare configs ]
Reported-by: NSteven Rostedt <rostedt@goodmis.org>
Signed-off-by: NRusty Russell <rusty@rustcorp.com.au>
LKML-Reference: <alpine.DEB.2.00.0904202113520.10097@gandalf.stny.rr.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/kernel/apic/nmi.c

@@ -414,7 +414,8 @@ nmi_watchdog_tick(struct pt_regs *regs, unsigned reason)
 		touched = 1;
 	}
 
-	if (cpumask_test_cpu(cpu, backtrace_mask)) {
+	/* We can be called before check_nmi_watchdog, hence NULL check. */
+	if (backtrace_mask != NULL && cpumask_test_cpu(cpu, backtrace_mask)) {
 		static DEFINE_SPINLOCK(lock);	/* Serialise the printks */
 
 		spin_lock(&lock);