tracing: Fix "integer as NULL pointer" warning.

kernel/trace/trace_output.c:256:24: warning: Using plain integer as NULL pointer

Signed-off-by: Thiago Farina <tfransosi@gmail.com>
LKML-Reference: <1264349038-1766-3-git-send-email-tfransosi@gmail.com>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>

kernel/trace/trace_output.c

@@ -253,7 +253,7 @@ void *trace_seq_reserve(struct trace_seq *s, size_t len)
 	void *ret;
 
 	if (s->full)
-		return 0;
+		return NULL;
 
 	if (len > ((PAGE_SIZE - 1) - s->len)) {
 		s->full = 1;