Use DECLARE_EVENT_CLASS, and save ~15K:

   text    data     bss     dec     hex filename
 171949   43028      48  215025   347f1 fs/xfs/linux-2.6/xfs_trace.o.orig
 156521   43028      36  199585   30ba1 fs/xfs/linux-2.6/xfs_trace.o

No change in functionality.
Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Acked-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Alex Elder <aelder@sgi.com>