From the syscall_fmts->arg entries for formatting strace-like syscalls.

This is when resolving the string "whence" on a filter expression for
the syscalls:sys_enter_lseek:

  Breakpoint 3, perf_evsel__syscall_arg_fmt (evsel=0xc91ed0, arg=0x7fffffff7cd0 "whence") at builtin-trace.c:3626
  3626	{
  (gdb) n
  3628		struct syscall_arg_fmt *fmt = __evsel__syscall_arg_fmt(evsel);
  (gdb) n
  3630		if (evsel->tp_format == NULL || fmt == NULL)
  (gdb) n
  3633		for (field = evsel->tp_format->format.fields; field; field = field->next, ++fmt)
  (gdb) n
  3634			if (strcmp(field->name, arg) == 0)
  (gdb) p field->name
  $3 = 0xc945e0 "__syscall_nr"
  (gdb) n
  3633		for (field = evsel->tp_format->format.fields; field; field = field->next, ++fmt)
  (gdb) p *fmt
  $4 = {scnprintf = 0x0, strtoul = 0x0, mask_val = 0x0, parm = 0x0, name = 0x0, nr_entries = 0, show_zero = false}
  (gdb) n
  3634			if (strcmp(field->name, arg) == 0)
  (gdb) p field->name
  $5 = 0xc94690 "fd"
  (gdb) n
  3633		for (field = evsel->tp_format->format.fields; field; field = field->next, ++fmt)
  (gdb) n
  3634			if (strcmp(field->name, arg) == 0)
  (gdb) n
  3633		for (field = evsel->tp_format->format.fields; field; field = field->next, ++fmt)
  (gdb) n
  3634			if (strcmp(field->name, arg) == 0)
  (gdb) p *fmt
  $9 = {scnprintf = 0x489be2 <syscall_arg__scnprintf_strarray>, strtoul = 0x0, mask_val = 0x0, parm = 0xa2da80 <strarray.whences>, name = 0x0,
    nr_entries = 0, show_zero = false}
  (gdb) p field->name
  $10 = 0xc947b0 "whence"
  (gdb) p fmt->parm
  $11 = (void *) 0xa2da80 <strarray.whences>
  (gdb) p *(struct strarray *)fmt->parm
  $12 = {offset = 0, nr_entries = 5, prefix = 0x724d37 "SEEK_", entries = 0xa2da40 <whences>}
  (gdb) p (struct strarray *)fmt->parm)->entries
  Junk after end of expression.
  (gdb) p ((struct strarray *)fmt->parm)->entries
  $13 = (const char **) 0xa2da40 <whences>
  (gdb) p ((struct strarray *)fmt->parm)->entries[0]
  $14 = 0x724d21 "SET"
  (gdb) p ((struct strarray *)fmt->parm)->entries[1]
  $15 = 0x724d25 "CUR"
  (gdb) p ((struct strarray *)fmt->parm)->entries[2]
  $16 = 0x724d29 "END"
  (gdb) p ((struct strarray *)fmt->parm)->entries[2]
  $17 = 0x724d29 "END"
  (gdb) p ((struct strarray *)fmt->parm)->entries[3]
  $18 = 0x724d2d "DATA"
  (gdb) p ((struct strarray *)fmt->parm)->entries[4]
  $19 = 0x724d32 "HOLE"
  (gdb)

Cc: Adrian Hunter <adrian.hunter@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-lc8h9jgvbnboe0g7ic8tra1y@git.kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>