rcu/nohz: Turn off tick for offloaded CPUs

rcu/nohz: Turn off tick for offloaded CPUs

Historically, no-CBs CPUs allowed the scheduler-clock tick to be
unconditionally disabled on any transition to idle or nohz_full userspace
execution (see the rcu_needs_cpu() implementations).  Unfortunately,
the checks used by rcu_needs_cpu() are defeated now that no-CBs CPUs
use ->cblist, which might make users of battery-powered devices rather
unhappy.  This commit therefore adds explicit rcu_segcblist_is_offloaded()
checks to return to the historical energy-efficient semantics.

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