tracing: Evaluate len expression only once in __dynamic_array macro

Use a temporary variable to store the expansion of the len expression.
If the evaluation is expensive, this commit will ensure it is evaluated
only once inside ftrace_get_offsets_<call>.

Link: http://lkml.kernel.org/r/1393651938-16418-3-git-send-email-filbranden@google.com

Signed-off-by: Filipe Brandenburger <filbranden@google.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

include/trace/ftrace.h

@@ -361,10 +361,11 @@ ftrace_define_fields_##call(struct ftrace_event_call *event_call)	\
 
 #undef __dynamic_array
 #define __dynamic_array(type, item, len)				\
+	__item_length = (len) * sizeof(type);				\
 	__data_offsets->item = __data_size +				\
 			       offsetof(typeof(*entry), __data);	\
-	__data_offsets->item |= ((len) * sizeof(type)) << 16;		\
-	__data_size += (len) * sizeof(type);
+	__data_offsets->item |= __item_length << 16;			\
+	__data_size += __item_length;
 
 #undef __string
 #define __string(item, src) __dynamic_array(char, item,			\
@@ -376,6 +377,7 @@ static inline notrace int ftrace_get_offsets_##call(			\
 	struct ftrace_data_offsets_##call *__data_offsets, proto)       \
 {									\
 	int __data_size = 0;						\
+	int __maybe_unused __item_length;				\
 	struct ftrace_raw_##call __maybe_unused *entry;			\
 									\
 	tstruct;							\