tracing: Fix for non-continuous cpu ids

Currently exception occures due to access beyond buffer_iter
range while using index of cpu bigger than num_possible_cpus().
Below there is an example for such exception when we use
cpus 0,1,16,17.

In order to fix buffer allocation size for non-continuous cpu ids
we allocate according to the max cpu id and not according to the
amount of possible cpus.

Example:
  $ cat /sys/kernel/debug/tracing/per_cpu/cpu1/trace
  Path: /bin/busybox
  CPU: 0 PID: 82 Comm: cat Not tainted 4.0.0 #29
  task: 80734c80 ti: 80012000 task.ti: 80012000

  [ECR   ]: 0x00220100 => Invalid Read @ 0x00000000 by insn @ 0x800abafc
  [EFA   ]: 0x00000000
  [BLINK ]: ring_buffer_read_finish+0x24/0x64
  [ERET  ]: rb_check_pages+0x20/0x188
  [STAT32]: 0x00001a00 :
  BTA: 0x800abafc  SP: 0x80013f0c  FP: 0x57719cf8
  LPS: 0x200036b4 LPE: 0x200036b8 LPC: 0x00000000
  r00: 0x8002aca0 r01: 0x00001606 r02: 0x00000000
  r03: 0x00000001 r04: 0x00000000 r05: 0x804b4954
  r06: 0x00030003 r07: 0x8002a260 r08: 0x00000286
  r09: 0x00080002 r10: 0x00001006 r11: 0x807351a4
  r12: 0x00000001

  Stack Trace:
    rb_check_pages+0x20/0x188
    ring_buffer_read_finish+0x24/0x64
    tracing_release+0x4e/0x170
    __fput+0x62/0x158
    task_work_run+0xa2/0xd4
    do_notify_resume+0x52/0x7c
    resume_user_mode_begin+0xdc/0xe0

Link: http://lkml.kernel.org/r/1433835155-6894-3-git-send-email-gilf@ezchip.comSigned-off-by: Noam Camus <noamc@ezchip.com>
Signed-off-by: Gil Fruchter <gilf@ezchip.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace.c

@@ -3035,7 +3035,7 @@ __tracing_open(struct inode *inode, struct file *file, bool snapshot)
 	if (!iter)
 		return ERR_PTR(-ENOMEM);
 
-	iter->buffer_iter = kcalloc(num_possible_cpus(), sizeof(*iter->buffer_iter),
+	iter->buffer_iter = kcalloc(nr_cpu_ids, sizeof(*iter->buffer_iter),
 				    GFP_KERNEL);
 	if (!iter->buffer_iter)
 		goto release;