rcuscale: Add kfree_rcu() single-argument scale test

rcuscale: Add kfree_rcu() single-argument scale test

The single-argument variant of kfree_rcu() is currently not
tested by any member of the rcutoture test suite.  This
commit therefore adds rcuscale code to test it.  This
testing is controlled by two new boolean module parameters,
kfree_rcu_test_single and kfree_rcu_test_double.  If one
is set and the other not, only the corresponding variant
is tested, otherwise both are tested, with the variant to
be tested determined randomly on each invocation.

Both of these module parameters are initialized to false,
so setting either to true will test only that variant.

Suggested-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>