Regression: partial revert "tracing: Remove lock_depth from event entry"

This partially reverts commit e6e1e2593592a8f6f6380496655d8c6f67431266.

That commit changed the structure layout of the trace structure, which
in turn broke PowerTOP (1.9x generation) quite badly.

I appreciate not wanting to expose the variable in question, and
PowerTOP was not using it, so I've replaced the variable with just a
padding field - that way if in the future a new field is needed it can
just use this padding field.

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
diff --git a/include/linux/ftrace_event.h b/include/linux/ftrace_event.h
index 22b32af..b5a550a 100644
--- a/include/linux/ftrace_event.h
+++ b/include/linux/ftrace_event.h
@@ -37,6 +37,7 @@
 	unsigned char		flags;
 	unsigned char		preempt_count;
 	int			pid;
+	int			padding;
 };
 
 #define FTRACE_MAX_EVENT						\