rcu/nocb: Reduce nocb_cb_wait() leaf rcu_node ->lock contention

Currently, nocb_cb_wait() advances callbacks on each pass through its
loop, though only if it succeeds in conditionally acquiring its leaf
rcu_node structure's ->lock.  Despite the conditional acquisition of
->lock, this does increase contention.  This commit therefore avoids
advancing callbacks unless there are callbacks in ->cblist whose grace
period has completed.

Note that nocb_cb_wait() doesn't worry about callbacks that have not
yet been assigned a grace period.  The idea is that the only reason for
nocb_cb_wait() to advance callbacks is to allow it to continue invoking
callbacks.  Time will tell whether this is the correct choice.

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