Docs/RCU/rculist_nulls: Assign 'obj' before use from the examples

Lookup example code snippets in rculist_nulls.rst are using 'obj'
without assignment.  Fix the code to assign it properly.

Signed-off-by: SeongJae Park <sj@kernel.org>
Reviewed-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>