tracing/filter: Dynamically allocate preds

For every filter that is made, we create predicates to hold every
operation within the filter. We have a max of 32 predicates that we
can hold. Currently, we allocate all 32 even if we only need to
use one.

Part of the reason we do this is that the filter can be used at
any moment by any event. Fortunately, the filter is only used
with preemption disabled. By reseting the count of preds used "n_preds"
to zero, then performing a synchronize_sched(), we can safely
free and reallocate a new array of preds.

Cc: Tom Zanussi <tzanussi@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace.h

@@ -661,7 +661,8 @@ struct ftrace_event_field {
 };
 
 struct event_filter {
-	int			n_preds;
+	int			n_preds;	/* Number assigned */
+	int			a_preds;	/* allocated */
 	struct filter_pred	**preds;
 	char			*filter_string;
 };