rcutorture: Increase rcu_read_delay() longdelay_ms

RCU now takes certain actions 100 and 200 milliseconds into a grace period
by default, but rcutorture only runs RCU read-side critical sections
with durations up to 50 milliseconds.  This commit therefore increases
test coverage by increasing the maximum critical-section duration to
300 milliseconds.  Note that the existing code automatically dials down
the probability of long delays based on the maximum duration, which means
that this change should not significantly change the rate of execution
of RCU read-side critical sections.

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