tracing/boottime: Fix kprobe event API usage

Fix boottime kprobe events to use API correctly for
multiple events.

For example, when we set a multiprobe kprobe events in
bootconfig like below,

  ftrace.event.kprobes.myevent {
  	probes = "vfs_read $arg1 $arg2", "vfs_write $arg1 $arg2"
  }

This cause an error;

  trace_boot: Failed to add probe: p:kprobes/myevent (null)  vfs_read $arg1 $arg2  vfs_write $arg1 $arg2

This shows the 1st argument becomes NULL and multiprobes
are merged to 1 probe.

Link: http://lkml.kernel.org/r/158779375766.6082.201939936008972838.stgit@devnote2

Cc: Ingo Molnar <mingo@kernel.org>
Cc: stable@vger.kernel.org
Fixes: 29a1548105 ("tracing: Change trace_boot to use kprobe_event interface")
Reviewed-by: Tom Zanussi <zanussi@kernel.org>
Signed-off-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace_boot.c

@@ -95,23 +95,19 @@ trace_boot_add_kprobe_event(struct xbc_node *node, const char *event)
 	struct xbc_node *anode;
 	char buf[MAX_BUF_LEN];
 	const char *val;
-	int ret;
-
-	kprobe_event_cmd_init(&cmd, buf, MAX_BUF_LEN);
-
-	ret = kprobe_event_gen_cmd_start(&cmd, event, NULL);
-	if (ret)
-		return ret;
+	int ret = 0;
 
 	xbc_node_for_each_array_value(node, "probes", anode, val) {
-		ret = kprobe_event_add_field(&cmd, val);
-		if (ret)
-			return ret;
-	}
+		kprobe_event_cmd_init(&cmd, buf, MAX_BUF_LEN);
 
-	ret = kprobe_event_gen_cmd_end(&cmd);
-	if (ret)
-		pr_err("Failed to add probe: %s\n", buf);
+		ret = kprobe_event_gen_cmd_start(&cmd, event, val);
+		if (ret)
+			break;
+
+		ret = kprobe_event_gen_cmd_end(&cmd);
+		if (ret)
+			pr_err("Failed to add probe: %s\n", buf);
+	}
 
 	return ret;
 }