修复clock_gettime返回类型错误，修复小时间间隔duration返回0问题 (#664)

* 修复clock_gettime返回类型错误，修正wtm初始化逻辑

* 修复duration在小时间间隔下为0的问题

* 临时修复时间流逝速度异常，在test-mount中加入运行时间检测

user/apps/test-mount/src/main.rs

@@ -1,7 +1,8 @@
 use core::ffi::{c_char, c_void};
 use libc::{mount, MS_BIND};
-
+use std::time;
 fn main() {
+    let clock = time::Instant::now();
     let source = b"\0".as_ptr() as *const c_char;
     let target = b"/mnt/tmp\0".as_ptr() as *const c_char;
     let fstype = b"ramfs\0".as_ptr() as *const c_char;
@@ -14,4 +15,6 @@ fn main() {
     } else {
         println!("Mount failed");
     }
+    let dur = clock.elapsed();
+    println!("mount costing time: {} ns", dur.as_nanos());
 }