perf trace: Support tracepoint dynamic char arrays

Things like:

  # grep __data_loc /sys/kernel/debug/tracing/events/sched/sched_process_exec/format
	field:__data_loc char[] filename;	offset:8;	size:4;	signed:1;
  #

That, at that offset (8) and with that size(8) have an integer that
contains the real length and offset for the contents of that array.

Now this works:

  # perf trace --max-events 1 -e sched:*exec -a
     0.000 sed/19441 sched:sched_process_exec(filename: "/usr/bin/sync", pid: 19441 (sync), old_pid: 19441 (sync))
  #

As when using the libtraceevent based beautifier:

  # perf trace --libtraceevent --max-events 1 -e sched:*exec -a
     0.000 sync/19463 sched:sched_process_exec(filename=/usr/bin/sync pid=19463 old_pid=19463)
  #

I.e. that 'filename' is implemented as a dynamic char array.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Luis Cláudio Gonçalves <lclaudio@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: https://lkml.kernel.org/n/tip-950p0m842fe6n7sxsdwqj5i2@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/trace/beauty/beauty.h

@@ -87,6 +87,7 @@ struct syscall_arg_fmt;
 
 /**
  * @val: value of syscall argument being formatted
+ * @len: for tracepoint dynamic arrays, if fmt->nr_entries == 0, then its not a fixed array, look at arg->len
  * @args: All the args, use syscall_args__val(arg, nth) to access one
  * @augmented_args: Extra data that can be collected, for instance, with eBPF for expanding the pathname for open, etc
  * @augmented_args_size: augmented_args total payload size
@@ -109,6 +110,7 @@ struct syscall_arg {
 	struct thread *thread;
 	struct trace  *trace;
 	void	      *parm;
+	u16	      len;
 	u8	      idx;
 	u8	      mask;
 	bool	      show_string_prefix;