rcu: Consolidate offline-CPU callback initialization

Currently, both rcu_cleanup_dead_cpu() and rcu_send_cbs_to_orphanage()
initialize the outgoing CPU's callback list.  However, only
rcu_cleanup_dead_cpu() invokes rcu_send_cbs_to_orphanage(), and
it does so unconditionally, which means that only one of these
initializations is required.  This commit therefore consolidates the
callback-list initialization with the rest of the callback handling in
rcu_send_cbs_to_orphanage().

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