rcu: Streamline code produced by __rcu_read_unlock()

This is a port of commit #be0e1e21 to TINY_PREEMPT_RCU.  This uses
noinline to prevent rcu_read_unlock_special() from being inlined into
__rcu_read_unlock().

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