tracing: Fix to use __always_unused attribute

____ftrace_check_##name() is used for compile-time check on
F_printk() only, so it should be marked as __unused instead
of __used.

Signed-off-by: Li Zefan <lizf@cn.fujitsu.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
LKML-Reference: <4AEE2D01.4010305@cn.fujitsu.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

kernel/trace/trace_export.c

@@ -48,11 +48,11 @@
 struct ____ftrace_##name {					\
 	tstruct							\
 };								\
-static void __used ____ftrace_check_##name(void)		\
+static void __always_unused ____ftrace_check_##name(void)	\
 {								\
 	struct ____ftrace_##name *__entry = NULL;		\
 								\
-	/* force cmpile-time check on F_printk() */		\
+	/* force compile-time check on F_printk() */		\
 	printk(print);						\
 }