perf trace: Ignore "overwrite" field if present in /events/header_page

That is not used in perf where we have the LOST events.

Without this patch we get:

[root@doppio ~]# perf lock report | head -3
  Warning: Error: expected 'data' but read 'overwrite'

So, to make the same perf command work with kernels with and without
this field, introduce variants for the parsing routines to not warn the
user in such case.

Discussed-with: Steven Rostedt <rostedt@goodmis.org>
Cc: Frédéric Weisbecker <fweisbec@gmail.com>
Cc: Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
LKML-Reference: <new-submission>
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>