bugfix: passthrough: Add MockCFileHandleWrapper

Previously, CFileHandle uses a *mut libc::c_char to transfer data
between user space and kernel space. The system call "name_to_handle_at"
will return the data with "copy_to_user". This may cause a bug because
the memory layout of CFileHandle fields may be noncontinuous with the
dynamically-sized array's. Therefore, the "copy_to_user" may destroy the
stack. This is reproduced on aarch64 only when using "opt-level=3" to
compile.

This commit adds three structs: __IncompleteArrayField, MockCFileHandle
and MockCFileHandleWrapper. We construct a dynamically-sized array with
continous memory address by using FamStruct trait, called
MockCFileHandleWrapper. It will manage the array automatically and
provide useful interfaces to help operate on array. We can thransfer
data during system call safely to protect the user stack.

Besides, we reuse the struct CFileHandle to save the information in Rust
version and provide four useful methods to be operated with
MockCFileHandleWrapper.

Fix clippy warning and update the unit test.

Signed-off-by: xuejun-xj <[email protected]>

src/abi/fuse_abi.rs

@@ -610,7 +610,7 @@ impl Attr {
             mode: st.st_mode,
             #[cfg(target_os = "macos")]
             mode: st.st_mode as u32,
-            nlink: st.st_nlink as u32,
+            nlink: st.st_nlink,
             uid: st.st_uid,
             gid: st.st_gid,
             rdev: st.st_rdev as u32,