rust: kernel: move `allocator` module under `alloc`

We will add more to the `alloc` module in subsequent patches (e.g.,
allocation flags and extension traits).
Reviewed-by: Benno Lossin <benno.lossin@proton.me>
Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>
Link: https://lore.kernel.org/r/20240328013603.206764-2-wedsonaf@gmail.comSigned-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/alloc.rs

+// SPDX-License-Identifier: GPL-2.0
+
+//! Extensions to the [`alloc`] crate.
+
+#[cfg(not(test))]
+#[cfg(not(testlib))]
+mod allocator;

rust/kernel/lib.rs

@@ -27,9 +27,7 @@
 // Allow proc-macros to refer to `::kernel` inside the `kernel` crate (this crate).
 extern crate self as kernel;
 
-#[cfg(not(test))]
-#[cfg(not(testlib))]
-mod allocator;
+pub mod alloc;
 mod build_assert;
 pub mod error;
 pub mod init;