perf script: Handle missing fields with -F +..

When using -F + syntax to add a field the existing defaults are
currently all marked user_set. This can cause errors when some field is
missing in the perf.data

This patch tracks the actually user set fields separately, so that we don't
error out in this case.

Before:

  % perf record true
  % perf script -F +metric
  Samples for 'cycles:ppp' event do not have CPU attribute set. Cannot print 'cpu' field.
  %

After:

  5 perf record true
  % perf script -F +metric
              perf 28936 278636.237688:          1 cycles:ppp:  ffffffff8117da99 perf_event_exec+0x59 (/lib/modules/4.20.0-odilo/build/vmlinux)
  ...
  %

Signed-off-by: Andi Kleen <ak@linux.intel.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Stephane Eranian <eranian@google.com>
Link: http://lkml.kernel.org/r/20190224153722.27020-2-andi@firstfloor.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>