rust: kernel: add LocalModule fallback for #[vtable] impls

Like commit 98f256e ("rust: doctest: add LocalModule fallback
for #[vtable] ThisModule"), add another `LocalModule` struct with a
null-pointer `ModuleMetadata` `impl` for the `kernel` crate, so that
`crate::LocalModule` (auto-inserted by `#[vtable]`) resolves correctly
when there is no `module!` macro.

This will be needed by DRM to use `#[vtable]` `impl` blocks in KUnit
tests within the `kernel` crate [1].

Signed-off-by: Danilo Krummrich <dakr@kernel.org>
Link: https://lore.kernel.org/rust-for-linux/DKNAS52KYWLD.M15VEC6U0F6R@kernel.org/ [1]
[ Created commit out of the diff in the link above. Fixed the
  `clippy::undocumented_unsafe_blocks` lint by wrapping with a block
  like in the other commit too. Added `#[allow(dead_code)]` until we
  actually (and unconditionally, i.e. KUnit tests may be not enabled)
  use it. - Miguel ]
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: Danilo Krummrich

rust/kernel/lib.rs

@@ -152,6 +152,20 @@ pub use uapi;
 /// Prefix to appear before log messages printed from within the `kernel` crate.
 const __LOG_PREFIX: &[u8] = b"rust_kernel\0";
 
+/// Dummy module type for `#[vtable]` `impl` blocks within the `kernel` crate (e.g. KUnit tests).
+// The `allow` is needed since it may be unused (e.g. KUnit tests may be disabled).
+#[allow(dead_code)]
+struct LocalModule;
+
+impl ModuleMetadata for LocalModule {
+    const NAME: &'static str::CStr = c"rust_kernel";
+
+    const THIS_MODULE: ThisModule = {
+        // SAFETY: `try_module_get`/`module_put` handle null module pointers gracefully.
+        unsafe { ThisModule::from_ptr(core::ptr::null_mut()) }
+    };
+}
+
 #[cfg(not(testlib))]
 #[panic_handler]
 fn panic(info: &core::panic::PanicInfo<'_>) -> ! {