rust: sync: make doctests compilable/testable

Rust documentation tests are going to be build/run-tested
with the KUnit integration added in a future patch, thus
update them to make them compilable/testable so that we
may start enforcing it.

Reviewed-by: Martin Rodriguez Reboredo <yakoyoku@gmail.com>
Reviewed-by: Björn Roy Baron <bjorn3_gh@protonmail.com>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: David Gow <davidgow@google.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>

rust/kernel/sync/lock/mutex.rs

@@ -63,6 +63,7 @@ macro_rules! new_mutex {
 /// assert_eq!(e.c, 10);
 /// assert_eq!(e.d.lock().a, 20);
 /// assert_eq!(e.d.lock().b, 30);
+/// # Ok::<(), Error>(())
 /// ```
 ///
 /// The following example shows how to use interior mutability to modify the contents of a struct

rust/kernel/sync/lock/spinlock.rs

@@ -61,6 +61,7 @@ macro_rules! new_spinlock {
 /// assert_eq!(e.c, 10);
 /// assert_eq!(e.d.lock().a, 20);
 /// assert_eq!(e.d.lock().b, 30);
+/// # Ok::<(), Error>(())
 /// ```
 ///
 /// The following example shows how to use interior mutability to modify the contents of a struct