perf ftrace: Fix access to pid in array when setting a pid filter

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@linux.ibm.com>
Cc: Sven Schnelle <svens@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Link: http://lore.kernel.org/lkml/20210421120400.2126433-1-tmricht@linux.ibm.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/builtin-ftrace.c

@@ -289,7 +289,7 @@ static int set_tracing_pid(struct perf_ftrace *ftrace)
 
 	for (i = 0; i < perf_thread_map__nr(ftrace->evlist->core.threads); i++) {
 		scnprintf(buf, sizeof(buf), "%d",
-			  ftrace->evlist->core.threads->map[i]);
+			  perf_thread_map__pid(ftrace->evlist->core.threads, i));
 		if (append_tracing_file("set_ftrace_pid", buf) < 0)
 			return -1;
 	}