tools/offcputime: Fix python3 string output

When using non-folded output, some of the strings would
be printed as raw byte arrays on python3, like this:

    b'finish_task_switch'
    b'__schedule'
    b'schedule'
    b'worker_thread'
    b'kthread'
    b'ret_from_fork'
    -                kworker/u16:1 (22022)
        2267942

This commit updates the code to treat these as utf8 strings,
consistent with all the other strings printed from this tool.

tools/offcputime.py

@@ -303,15 +303,15 @@ def signal_ignore(signal, frame):
  print(" [Missed Kernel Stack]")
  else:
  for addr in kernel_stack:
- print(" %s" % b.ksym(addr))
+ print(" %s" % b.ksym(addr).decode('utf-8', 'replace'))
  if not args.kernel_stacks_only:
  if need_delimiter and k.user_stack_id >= 0 and k.kernel_stack_id >= 0:
  print(" --")
  if stack_id_err(k.user_stack_id):
  print(" [Missed User Stack]")
  else:
  for addr in user_stack:
- print(" %s" % b.sym(addr, k.tgid))
+ print(" %s" % b.sym(addr, k.tgid).decode('utf-8', 'replace'))
  print(" %-16s %s (%d)" % ("-", k.name.decode('utf-8', 'replace'), k.pid))
  print(" %d\n" % v.value)