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.com
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

rust/kernel/alloc.rs

@@ -0,0 +1,7 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Extensions to the [`alloc`] crate.
+
+#[cfg(not(test))]
+#[cfg(not(testlib))]
+mod allocator;