rcu: Allow processing deferred QSes for exiting RCU-preempt readers

If an RCU-preempt read-side critical section is exiting, that is,
->rcu_read_lock_nesting is negative, then it is a good time to look
at the possibility of reporting deferred quiescent states.  This
commit therefore updates the checks in rcu_preempt_need_deferred_qs()
to allow exiting critical sections to report deferred quiescent states.

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