perf sched: Move curr_thread initialization to perf_sched__map()

The curr_thread is used only for the 'perf sched map'. Put initialization
in perf_sched__map() to reduce unnecessary actions in other commands.

Simple functional testing:

  # perf sched record perf bench sched messaging
  # Running 'sched/messaging' benchmark:
  # 20 sender and receiver processes per group
  # 10 groups == 400 processes run

       Total time: 0.197 [sec]
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 15.526 MB perf.data (140095 samples) ]

  # perf sched map
    *A0                                                               451264.532445 secs A0 => migration/0:15
    *.                                                                451264.532468 secs .  => swapper:0
     .  *B0                                                           451264.532537 secs B0 => migration/1:21
     .  *.                                                            451264.532560 secs
     .   .  *C0                                                       451264.532644 secs C0 => migration/2:27
     .   .  *.                                                        451264.532668 secs
     .   .   .  *D0                                                   451264.532753 secs D0 => migration/3:33
     .   .   .  *.                                                    451264.532778 secs
     .   .   .   .  *E0                                               451264.532861 secs E0 => migration/4:39
     .   .   .   .  *.                                                451264.532886 secs
     .   .   .   .   .  *F0                                           451264.532973 secs F0 => migration/5:45
  <SNIP>
     A7  A7  A7  A7  A7 *A7  .   .   .   .   .   .   .   .   .   .    451264.790785 secs
     A7  A7  A7  A7  A7  A7 *A7  .   .   .   .   .   .   .   .   .    451264.790858 secs
     A7  A7  A7  A7  A7  A7  A7 *A7  .   .   .   .   .   .   .   .    451264.790934 secs
     A7  A7  A7  A7  A7  A7  A7  A7 *A7  .   .   .   .   .   .   .    451264.791004 secs
     A7  A7  A7  A7  A7  A7  A7  A7  A7 *A7  .   .   .   .   .   .    451264.791075 secs
     A7  A7  A7  A7  A7  A7  A7  A7  A7  A7 *A7  .   .   .   .   .    451264.791143 secs
     A7  A7  A7  A7  A7  A7  A7  A7  A7  A7  A7 *A7  .   .   .   .    451264.791232 secs
     A7  A7  A7  A7  A7  A7  A7  A7  A7  A7  A7  A7 *A7  .   .   .    451264.791336 secs
     A7  A7  A7  A7  A7  A7  A7  A7  A7  A7  A7  A7  A7 *A7  .   .    451264.791407 secs
     A7  A7  A7  A7  A7  A7  A7  A7  A7  A7  A7  A7  A7  A7 *A7  .    451264.791484 secs
     A7  A7  A7  A7  A7  A7  A7  A7  A7  A7  A7  A7  A7  A7  A7 *A7   451264.791553 secs
  # echo $?
  0

Signed-off-by: Yang Jihong <yangjihong1@huawei.com>
Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Link: https://lore.kernel.org/r/20240206083228.172607-4-yangjihong1@huawei.com