rcu: Consolidate expedited GP code into exp_funnel_lock()

This commit pulls the grace-period-start counter adjustment and tracing
from synchronize_rcu_expedited() and synchronize_sched_expedited()
into exp_funnel_lock(), thus eliminating some code duplication.

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