rcu: Modulate grace-period slow init to normalize delay

Currently, the larger the gp_init_delay boot parameter, the slower
rcutorture will sequence through grace periods.  This commit avoids this
issue by decreasing the probability of slowing initialization of a given
grace period as the degree of slowness increases.

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