rcu: Make RCU_FAST_NO_HZ respect nohz= boot parameter

If the nohz= boot parameter disables nohz, then RCU_FAST_NO_HZ needs to
also disable itself.  This commit therefore checks for tick_nohz_enabled
being zero, disabling rcu_prepare_for_idle() if so.  This commit assumes
that tick_nohz_enabled can change at runtime: If this is not the case,
then a simpler approach suffices.

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