rust: page: use the "kernel vertical" imports style

Convert the imports to use the "kernel vertical" imports style [1].

No functional changes intended.

Link: https://docs.kernel.org/rust/coding-guidelines.html#imports [1]
Signed-off-by: Andreas Hindborg <a.hindborg@kernel.org>
Link: https://patch.msgid.link/20260604-unique-ref-v17-4-7b4c3d2930b9@kernel.org
[ Picked from larger series and reworded. Adjusted the `error::`
  block too. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/page.rs

@@ -3,17 +3,25 @@
 //! Kernel page allocation and management.
 
 use crate::{
-    alloc::{AllocError, Flags},
+    alloc::{
+        AllocError,
+        Flags, //
+    },
     bindings,
-    error::code::*,
-    error::Result,
-    uaccess::UserSliceReader,
+    error::{
+        code::*,
+        Result, //
+    },
+    uaccess::UserSliceReader, //
 };
 use core::{
     marker::PhantomData,
     mem::ManuallyDrop,
     ops::Deref,
-    ptr::{self, NonNull},
+    ptr::{
+        self,
+        NonNull, //
+    }, //
 };
 
 /// A bitwise shift for the page size.