sched: do not use cpu_to_node() to find an offlined cpu's node.

sched: do not use cpu_to_node() to find an offlined cpu's node.

If a cpu is offline, its nid will be set to -1, and cpu_to_node(cpu) will
return -1.  As a result, cpumask_of_node(nid) will return NULL.  In this
case, find_next_bit() in for_each_cpu will get a NULL pointer and cause
panic.

Here is a call trace:
[  609.824017] Call Trace:
[  609.824017]  <IRQ>
[  609.824017]  [<ffffffff810b0721>] select_fallback_rq+0x71/0x190
[  609.824017]  [<ffffffff810b086e>] ? try_to_wake_up+0x2e/0x2f0
[  609.824017]  [<ffffffff810b0b0b>] try_to_wake_up+0x2cb/0x2f0
[  609.824017]  [<ffffffff8109da08>] ? __run_hrtimer+0x78/0x320
[  609.824017]  [<ffffffff810b0b85>] wake_up_process+0x15/0x20
[  609.824017]  [<ffffffff8109ce62>] hrtimer_wakeup+0x22/0x30
[  609.824017]  [<ffffffff8109da13>] __run_hrtimer+0x83/0x320
[  609.824017]  [<ffffffff8109ce40>] ? update_rmtp+0x80/0x80
[  609.824017]  [<ffffffff8109df56>] hrtimer_interrupt+0x106/0x280
[  609.824017]  [<ffffffff810a72c8>] ? sd_free_ctl_entry+0x68/0x70
[  609.824017]  [<ffffffff8167cf39>] smp_apic_timer_interrupt+0x69/0x99
[  609.824017]  [<ffffffff8167be2f>] apic_timer_interrupt+0x6f/0x80

There is a hrtimer process sleeping, whose cpu has already been offlined.
When it is waken up, it tries to find another cpu to run, and get a -1
nid.  As a result, cpumask_of_node(-1) returns NULL, and causes ernel
panic.

This patch fixes this problem by judging if the nid is -1.  If nid is not
-1, a cpu on the same node will be picked.  Else, a online cpu on another
node will be picked.

Signed-off-by: Tang Chen <tangchen@cn.fujitsu.com>
Signed-off-by: Wen Congyang <wency@cn.fujitsu.com>
Cc: Yasuaki Ishimatsu <isimatu.yasuaki@jp.fujitsu.com>
Cc: David Rientjes <rientjes@google.com>
Cc: Jiang Liu <liuj97@gmail.com>
Cc: Minchan Kim <minchan.kim@gmail.com>
Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>