ocfs2: Add ocfs2_trace.h.

About one year ago, Wengang Wang tried some first steps
to add tracepoints to ocfs2. Hiss original patch is here:
http://oss.oracle.com/pipermail/ocfs2-devel/2009-November/005512.html

But as Steven Rostedt indicated in his article
http://lwn.net/Articles/383362/, we'd better have our trace
files resides in fs/ocfs2, so I rewrited the patch using the
method Steven mentioned in that article.

Signed-off-by: Wengang Wang <wen.gang.wang@oracle.com>
Signed-off-by: Tao Ma <boyu.mt@taobao.com>