rcu: Initialize tiny RCU stall-warning timeouts at boot

The current tiny RCU stall-warning code assumes that the jiffies counter
starts at zero, however, it is sometimes initialized to other values,
for example, -30,000.  This commit therefore changes rcu_init() to
invoke reset_cpu_stall_ticks() for both flavors of RCU to initialize
the stall-warning times properly at boot.

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