tracing/function-graph-tracer: trace the idle tasks

When the function graph tracer is activated, it iterates over the task_list
to allocate a stack to store the return addresses.

But the per cpu idle tasks are not iterated by using
do_each_thread / while_each_thread.

So we have to iterate on them manually.

This fixes somes weirdness in the traces and many losses of traces.
Examples on two cpus:

 0)   Xorg-4287    |   2.906 us    |              }
 0)   Xorg-4287    |   3.965 us    |            }
 0)   Xorg-4287    |   5.302 us    |          }
 ------------------------------------------
 0)   Xorg-4287    =>    <idle>-0
 ------------------------------------------

 0)    <idle>-0    |   2.861 us    |                        }
 0)    <idle>-0    |   0.526 us    |                        set_normalized_timespec();
 0)    <idle>-0    |   7.201 us    |                      }
 0)    <idle>-0    |   8.214 us    |                    }
 0)    <idle>-0    |               |                    clockevents_program_event() {
 0)    <idle>-0    |               |                      lapic_next_event() {
 0)    <idle>-0    |   0.510 us    |                        native_apic_mem_write();
 0)    <idle>-0    |   1.546 us    |                      }
 0)    <idle>-0    |   2.583 us    |                    }
 0)    <idle>-0    | + 12.435 us   |                  }
 0)    <idle>-0    | + 13.470 us   |                }
 0)    <idle>-0    |   0.608 us    |                _spin_unlock_irqrestore();
 0)    <idle>-0    | + 23.270 us   |              }
 0)    <idle>-0    | + 24.336 us   |            }
 0)    <idle>-0    | + 25.417 us   |          }
 0)    <idle>-0    |   0.593 us    |          _spin_unlock();
 0)    <idle>-0    | + 41.869 us   |        }
 0)    <idle>-0    | + 42.906 us   |      }
 0)    <idle>-0    | + 95.035 us   |    }
 0)    <idle>-0    |   0.540 us    |    menu_reflect();
 0)    <idle>-0    | ! 100.404 us  |  }
 0)    <idle>-0    |   0.564 us    |  mce_idle_callback();
 0)    <idle>-0    |               |  enter_idle() {
 0)    <idle>-0    |   0.526 us    |    mce_idle_callback();
 0)    <idle>-0    |   1.757 us    |  }
 0)    <idle>-0    |               |  cpuidle_idle_call() {
 0)    <idle>-0    |               |    menu_select() {
 0)    <idle>-0    |   0.525 us    |      pm_qos_requirement();
 0)    <idle>-0    |   0.518 us    |      tick_nohz_get_sleep_length();
 0)    <idle>-0    |   2.621 us    |    }
[...]
 1)    <idle>-0    |   0.518 us    |              touch_softlockup_watchdog();
 1)    <idle>-0    | + 14.355 us   |            }
 1)    <idle>-0    | + 22.840 us   |          }
 1)    <idle>-0    | + 25.949 us   |        }
 1)    <idle>-0    |               |        handle_irq() {
 1)    <idle>-0    |   0.511 us    |          irq_to_desc();
 1)    <idle>-0    |               |          handle_edge_irq() {
 1)    <idle>-0    |   0.638 us    |            _spin_lock();
 1)    <idle>-0    |               |            ack_apic_edge() {
 1)    <idle>-0    |   0.510 us    |              irq_to_desc();
 1)    <idle>-0    |               |              move_native_irq() {
 1)    <idle>-0    |   0.510 us    |                irq_to_desc();
 1)    <idle>-0    |   1.532 us    |              }
 1)    <idle>-0    |   0.511 us    |              native_apic_mem_write();
 ------------------------------------------
 1)    <idle>-0    =>    cat-5073
 ------------------------------------------

 1)    cat-5073    |   3.731 us    |                    }
 1)    cat-5073    |               |                    run_local_timers() {
 1)    cat-5073    |   0.533 us    |                      hrtimer_run_queues();
 1)    cat-5073    |               |                      raise_softirq() {
 1)    cat-5073    |               |                        __raise_softirq_irqoff() {
 1)    cat-5073    |               |                          /* nr: 1 */
 1)    cat-5073    |   2.718 us    |                        }
 1)    cat-5073    |   3.814 us    |                      }

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>