perf tools: Fix proper buffer size for feature processing

After Song Liu's segfault fix for pipe mode, Arnaldo reported following
error:

  # perf record -o - | perf script
  0x514 [0x1ac]: failed to process type: 80

It's caused by wrong buffer size setup in feature processing, which
makes cpu topology feature fail, because it's using buffer size to
recognize its header version.
Reported-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: David Carrillo-Cisneros <davidcc@google.com>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Song Liu <songliubraving@fb.com>
Fixes: e9def1b2 ("perf tools: Add feature header record to pipe-mode")
Link: http://lkml.kernel.org/r/20190715140426.32509-1-jolsa@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/util/header.c

@@ -3747,7 +3747,7 @@ int perf_event__process_feature(struct perf_session *session,
 		return 0;
 
 	ff.buf  = (void *)fe->data;
-	ff.size = event->header.size - sizeof(event->header);
+	ff.size = event->header.size - sizeof(*fe);
 	ff.ph = &session->header;
 
 	if (feat_ops[feat].process(&ff, NULL))