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Command 'perf ftrace -v -- ls' fails in s390 (at least 5.12.0rc6).

The root cause is a missing pointer dereference which causes an
array element address to be used as PID.

Fix this by extracting the PID.

Output before:
  # ./perf ftrace -v -- ls
  function_graph tracer is used
  write '-263732416' to tracing/set_ftrace_pid failed: Invalid argument
  failed to set ftrace pid
  #

Output after:
   ./perf ftrace -v -- ls
   function_graph tracer is used
   # tracer: function_graph
   #
   # CPU  DURATION                  FUNCTION CALLS
   # |     |   |                     |   |   |   |
   4)               |  rcu_read_lock_sched_held() {
   4)   0.552 us    |    rcu_lockdep_current_cpu_online();
   4)   6.124 us    |  }
Reported-by: Alexander Schmidt <alexschm@de.ibm.com>
Signed-off-by: Thomas Richter <tmricht@linux.ibm.com>
Acked-by: Namhyung Kim <namhyung@kernel.org>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Sumanth Korikkar <sumanthk@li...