rcu/nocb: Consolidate rcu_nocb_cpu_offload/deoffload functions

rcu/nocb: Consolidate rcu_nocb_cpu_offload/deoffload functions

The rcu_nocb_cpu_offload() and rcu_nocb_cpu_deoffload() functions are
nearly duplicates.

Therefore, extract the common logic into rcu_nocb_cpu_toggle_offload()
which takes an 'offload' boolean, and make both exported functions
simple wrappers.

This eliminates a bunch of duplicate code at the call sites, namely
mutex locking, CPU hotplug locking and CPU online checks.

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>