rcu/nocb: Use build-time no-CBs check in rcu_core()

rcu/nocb: Use build-time no-CBs check in rcu_core()

Currently, rcu_core() invokes rcu_segcblist_is_offloaded() each time it
needs to know whether the current CPU is a no-CBs CPU.  Given that it is
not possible to change the no-CBs status of a CPU after boot, and given
that it is not possible to even have no-CBs CPUs in CONFIG_RCU_NOCB_CPU=n
kernels, this repeated runtime invocation wastes CPU.  This commit
therefore created a const on-stack variable to allow this check to be
done only once per rcu_core() invocation.

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