rcu/nocb: Add an option to offload all CPUs on boot

Systems built with CONFIG_RCU_NOCB_CPU=y but booted without either
the rcu_nocbs= or rcu_nohz_full= kernel-boot parameters will not have
callback offloading on any of the CPUs, nor can any of the CPUs be
switched to enable callback offloading at runtime.  Although this is
intentional, it would be nice to have a way to offload all the CPUs
without having to make random bootloaders specify either the rcu_nocbs=
or the rcu_nohz_full= kernel-boot parameters.

This commit therefore provides a new CONFIG_RCU_NOCB_CPU_DEFAULT_ALL
Kconfig option that switches the default so as to offload callback
processing on all of the CPUs.  This default can still be overridden
using the rcu_nocbs= and rcu_nohz_full= kernel-boot parameters.

Reviewed-by: Kalesh Singh <[email protected]>
Reviewed-by: Uladzislau Rezki <[email protected]>
(In v4.1, fixed issues with CONFIG maze reported by kernel test robot).
Reported-by: kernel test robot <[email protected]>
Signed-off-by: Joel Fernandes <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>
Reviewed-by: Neeraj Upadhyay <[email protected]>

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