Use nanoseconds in filestat

This commit fixes the `filestat` type which contains three timestamps.
Per [WASI spec], they should be in nanoseconds.  This is only a problem
for directories.

[WASI spec]: https://github.com/WebAssembly/WASI/blob/main/legacy/preview1/docs.md#-timestamp-u64

Signed-off-by: Yage Hu <[email protected]>

lib/virtual-fs/src/mem_fs/mod.rs

@@ -142,7 +142,7 @@ fn time() -> u64 {
         std::time::SystemTime::now()
             .duration_since(std::time::SystemTime::UNIX_EPOCH)
             .unwrap()
-            .as_secs()
+            .as_nanos() as u64
     }
 
     #[cfg(feature = "no-time")]