rcu: Allow rcutorture's stat_interval parameter to be changed at runtime

When rcutorture is compiled directly into the kernel
(instead of separately as a module), it is necessary to specify
rcutorture.stat_interval as a kernel command-line parameter, otherwise,
the rcu_torture_stats kthread is never started.  However, when working
with the system after it has booted, it is convenient to be able to
change the time between statistic printing, particularly when logged
into the console.

This commit therefore allows the stat_interval parameter to be changed
at runtime.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>