rcutorture: Abstract rcu_torture_random()

Because rcu_torture_random() will be used by the locking equivalent to
rcutorture, pull it out into its own module.  This new module cannot
be separately configured, instead, use the Kconfig "select" statement
from the Kconfig options of tests depending on it.

Suggested-by: Rusty Russell <rusty@rustcorp.com.au>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>