rcu: Explicitly initialize RCU_FAST_NO_HZ per-CPU variables

The current initialization of the RCU_FAST_NO_HZ per-CPU variables makes
needless and fragile assumptions about the initial value of things like
the jiffies counter.  This commit therefore explicitly initializes all of
them that are better started with a non-zero value.  It also adds some
comments describing the per-CPU state variables.

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