rust: fs: add file::Offset type alias

rust: fs: add file::Offset type alias

Add a type alias for file offsets, i.e. bindings::loff_t. Trying to
avoid using raw bindings types, this seems to be the better alternative
compared to just using i64.

Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Link: https://patch.msgid.link/20251020222722.240473-2-dakr@kernel.org
Signed-off-by: Danilo Krummrich <dakr@kernel.org>