tracing/boot: Fix an IS_ERR() vs NULL bug

The trace_array_get_by_name() function doesn't return error pointers,
it returns NULL on error.

Link: http://lkml.kernel.org/r/20200117053007.5h2juv272pokqhtq@kili.mountain

Fixes: 4f712a4d ("tracing/boot: Add instance node support")
Acked-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace_boot.c

@@ -322,7 +322,7 @@ trace_boot_init_instances(struct xbc_node *node)
 			continue;
 
 		tr = trace_array_get_by_name(p);
-		if (IS_ERR(tr)) {
+		if (!tr) {
 			pr_err("Failed to get trace instance %s\n", p);
 			continue;
 		}