rcutorture: Make object_debug also double call_rcu() heap object

rcutorture: Make object_debug also double call_rcu() heap object

This commit provides a test for call_rcu() printing the allocation address
of a double-freed callback by double-freeing a callback allocated via
kmalloc().  However, this commit does not depend on any other commit.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>