rust: pid_namespace: update AlwaysRefCounted imports from sync::aref

rust: pid_namespace: update AlwaysRefCounted imports from sync::aref

Update call sites in `pid_namespace.rs` to import
`AlwaysRefCounted` from `sync::aref` instead of `types`.

This aligns with the ongoing effort to move `ARef` and
`AlwaysRefCounted` to `sync`.

Suggested-by: Benno Lossin <lossin@kernel.org>
Link: https://github.com/Rust-for-Linux/linux/issues/1173
Signed-off-by: Shankari Anand <shankari.ak0208@gmail.com>
Link: https://lore.kernel.org/20250816122323.11657-1-shankari.ak0208@gmail.com
Reviewed-by: Benno Lossin <lossin@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>