sched: fix kernel warning on /proc/sched_debug access

Luis Henriques reported that with CONFIG_PREEMPT=y + CONFIG_PREEMPT_DEBUG=y +
CONFIG_SCHED_DEBUG=y + CONFIG_LATENCYTOP=y enabled, the following warning
triggers when using latencytop:

> [  775.663239] BUG: using smp_processor_id() in preemptible [00000000] code: latencytop/6585
> [  775.663303] caller is native_sched_clock+0x3a/0x80
> [  775.663314] Pid: 6585, comm: latencytop Tainted: G        W 2.6.28-rc4-00355-g9c7c354 #1
> [  775.663322] Call Trace:
> [  775.663343]  [<ffffffff803a94e4>] debug_smp_processor_id+0xe4/0xf0
> [  775.663356]  [<ffffffff80213f7a>] native_sched_clock+0x3a/0x80
> [  775.663368]  [<ffffffff80213e19>] sched_clock+0x9/0x10
> [  775.663381]  [<ffffffff8024550d>] proc_sched_show_task+0x8bd/0x10e0
> [  775.663395]  [<ffffffff8034466e>] sched_show+0x3e/0x80
> [  775.663408]  [<ffffffff8031039b>] seq_read+0xdb/0x350
> [  775.663421]  [<ffffffff80368776>] ? security_file_permission+0x16/0x20
> [  775.663435]  [<ffffffff802f4198>] vfs_read+0xc8/0x170
> [  775.663447]  [<ffffffff802f4335>] sys_read+0x55/0x90
> [  775.663460]  [<ffffffff8020c67a>] system_call_fastpath+0x16/0x1b
> ...

This breakage was caused by me via:

  7cbaef9: sched: optimize sched_clock() a bit

Change the calls to cpu_clock().

Reported-by: Luis Henriques <henrix@sapo.pt>

kernel/sched_debug.c

@@ -423,10 +423,11 @@ void proc_sched_show_task(struct task_struct *p, struct seq_file *m)
 #undef __P
 
 	{
+		unsigned int this_cpu = raw_smp_processor_id();
 		u64 t0, t1;
 
-		t0 = sched_clock();
-		t1 = sched_clock();
+		t0 = cpu_clock(this_cpu);
+		t1 = cpu_clock(this_cpu);
 		SEQ_printf(m, "%-35s:%21Ld\n",
 			   "clock-delta", (long long)(t1-t0));
 	}