tracing: Use IS_ERR() check for return value of tracing_init_dentry()

tracing_init_dentry() will soon return NULL as a valid pointer for the
top level tracing directroy. NULL can not be used as an error value.
Instead, switch to ERR_PTR() and check the return status with
IS_ERR().

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>
diff --git a/kernel/trace/trace_functions_graph.c b/kernel/trace/trace_functions_graph.c
index ba47600..2d25ad1 100644
--- a/kernel/trace/trace_functions_graph.c
+++ b/kernel/trace/trace_functions_graph.c
@@ -1437,7 +1437,7 @@
 	struct dentry *d_tracer;
 
 	d_tracer = tracing_init_dentry();
-	if (!d_tracer)
+	if (IS_ERR(d_tracer))
 		return 0;
 
 	trace_create_file("max_graph_depth", 0644, d_tracer,