rust: file: mark LocalFile as repr(transparent)

Unsafe code in `LocalFile`'s methods assumes that the type has the same
layout as the inner `bindings::file`. This is not guaranteed by the default
struct representation in Rust, but requires specifying the `transparent`
representation.

The `File` struct (which also wraps `bindings::file`) is already marked as
`repr(transparent)`, so this change makes their layouts equivalent.

Fixes: 8518498 ("rust: file: add Rust abstraction for `struct file`")
Closes: Rust-for-Linux#1165
Signed-off-by: Pekka Ristola <[email protected]>
Link: https://lore.kernel.org/[email protected]
Reviewed-by: Benno Lossin <[email protected]>
Reviewed-by: Alice Ryhl <[email protected]>
Signed-off-by: Christian Brauner <[email protected]>

Author: pekkarr

rust/kernel/fs/file.rs

@@ -219,6 +219,7 @@ unsafe impl AlwaysRefCounted for File {
 ///   must be on the same thread as this file.
 ///
 /// [`assume_no_fdget_pos`]: LocalFile::assume_no_fdget_pos
+#[repr(transparent)]
 pub struct LocalFile {
     inner: Opaque<bindings::file>,
 }