perf tools: Add __data_loc support

This patch is required to test the next patch for perf lock.

At 064739bc ,
support for the modifier "__data_loc" of format is added.

But, when I wanted to parse format of lock_acquired (or some
event else), raw_field_ptr() did not returned correct pointer.

So I modified raw_field_ptr() like this patch. Then
raw_field_ptr() works well.

Signed-off-by: Hitoshi Mitake <mitake@dcl.info.waseda.ac.jp>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Tom Zanussi <tzanussi@gmail.com>
Cc: Steven Rostedt <srostedt@redhat.com>
LKML-Reference: <1264851813-8413-2-git-send-email-mitake@dcl.info.waseda.ac.jp>
[ v3: fixed minor stylistic detail ]
Signed-off-by: Ingo Molnar <mingo@elte.hu>