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: Rust-for-Linux#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>

Author: Shankari02

rust/kernel/pid_namespace.rs

@@ -7,10 +7,7 @@
 //! C header: [`include/linux/pid_namespace.h`](srctree/include/linux/pid_namespace.h) and
 //! [`include/linux/pid.h`](srctree/include/linux/pid.h)
 
-use crate::{
-    bindings,
-    types::{AlwaysRefCounted, Opaque},
-};
+use crate::{bindings, sync::aref::AlwaysRefCounted, types::Opaque};
 use core::ptr;
 
 /// Wraps the kernel's `struct pid_namespace`. Thread safe.