mountns: potential fd leak when readlinkat call

when mntns_collect_root->readlinkat call faild, we should close pdf
Signed-off-by: Libo Chen <libo.chen@huawei.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

mount.c

@@ -787,8 +787,10 @@ int mntns_collect_root(pid_t pid)
 
 	pfd = open_pid_proc(pid);
 	ret = readlinkat(pfd, "root", path, sizeof(path) - 1);
-	if (ret < 0)
+	if (ret < 0){
+		close_pid_proc();
 		return ret;
+	}
 
 	path[ret] = '\0';