tests: use standard malloc/free for synchronize_rcu()

Allows removing mutex from tests, which allow testing scalability of
concurrent synchronize_rcu() executions.

CC: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
CC: Lai Jiangshan <laijs@cn.fujitsu.com>
CC: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>