tracing: Use __printf markup to silence compiler

Silence warnings (triggered at W=1) by adding relevant __printf attributes.

  CC      kernel/trace/trace.o
kernel/trace/trace.c: In function ‘__trace_array_vprintk’:
kernel/trace/trace.c:2979:2: warning: function might be possible candidate for ‘gnu_printf’ format attribute [-Wsuggest-attribute=format]
  len = vscnprintf(tbuffer, TRACE_BUF_SIZE, fmt, args);
  ^~~
  AR      kernel/trace/built-in.o

Link: http://lkml.kernel.org/r/20180308205843.27447-1-malat@debian.org

Signed-off-by: Mathieu Malaterre <malat@debian.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace.c

@@ -2953,6 +2953,7 @@ out_nobuffer:
 }
 EXPORT_SYMBOL_GPL(trace_vbprintk);
 
+__printf(3, 0)
 static int
 __trace_array_vprintk(struct ring_buffer *buffer,
 		      unsigned long ip, const char *fmt, va_list args)
@@ -3007,12 +3008,14 @@ out_nobuffer:
 	return len;
 }
 
+__printf(3, 0)
 int trace_array_vprintk(struct trace_array *tr,
 			unsigned long ip, const char *fmt, va_list args)
 {
 	return __trace_array_vprintk(tr->trace_buffer.buffer, ip, fmt, args);
 }
 
+__printf(3, 0)
 int trace_array_printk(struct trace_array *tr,
 		       unsigned long ip, const char *fmt, ...)
 {
@@ -3028,6 +3031,7 @@ int trace_array_printk(struct trace_array *tr,
 	return ret;
 }
 
+__printf(3, 4)
 int trace_array_printk_buf(struct ring_buffer *buffer,
 			   unsigned long ip, const char *fmt, ...)
 {
@@ -3043,6 +3047,7 @@ int trace_array_printk_buf(struct ring_buffer *buffer,
 	return ret;
 }
 
+__printf(2, 0)
 int trace_vprintk(unsigned long ip, const char *fmt, va_list args)
 {
 	return trace_array_vprintk(&global_trace, ip, fmt, args);