rcu/nocb: Advance CBs after merge in rcutree_migrate_callbacks()

The rcutree_migrate_callbacks() invokes rcu_advance_cbs() on both the
offlined CPU's ->cblist and that of the surviving CPU, then merges
them.  However, after the merge, and of the offlined CPU's callbacks
that were not ready to be invoked will no longer be associated with a
grace-period number.  This commit therefore invokes rcu_advance_cbs()
one more time on the merged ->cblist in order to assign a grace-period
number to these callbacks.

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