srcu: Crude control of expedited grace periods

SRCU's implementation of expedited grace periods has always assumed
that the SRCU instance is idle when the expedited request arrives.
This commit improves this a bit by maintaining a count of the number
of outstanding expedited requests, thus allowing prior non-expedited
grace periods accommodate these requests by shifting to expedited mode.
However, any non-expedited wait already in progress will still wait for
the full duration.

Improved control of expedited grace periods is planned, but one step
at a time.

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

include/linux/srcu.h

@@ -42,6 +42,7 @@ struct srcu_array {
 struct srcu_struct {
 	unsigned long completed;
 	unsigned long srcu_gp_seq;
+	atomic_t srcu_exp_cnt;
 	struct srcu_array __percpu *per_cpu_ref;
 	spinlock_t queue_lock; /* protect ->srcu_cblist */
 	struct rcu_segcblist srcu_cblist;