rcu: Make synchronize_sched_expedited() call wait_rcu_gp()

Currently, synchronize_sched_expedited() will call synchronize_sched()
if there is danger of counter wrap.  But if configuration says to
always do expedited grace periods, synchronize_sched() will just
call synchronize_sched_expedited() right back again.  In theory,
the old expedited operations will complete, the counters will
get back in synch, and the recursion will end.  But we could
easily run out of stack long before that time.  This commit
therefore makes synchronize_sched_expedited() invoke the underlying
wait_rcu_gp(call_rcu_sched) instead of synchronize_sched(), the same as
all the other calls out from synchronize_sched_expedited().

This bug was introduced by commit 1924bcb (Avoid counter wrap in
synchronize_sched_expedited()).

Reported-by: Rik van Riel <riel@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

kernel/rcu/tree.c

@@ -3290,7 +3290,7 @@ void synchronize_sched_expedited(void)
 	if (ULONG_CMP_GE((ulong)atomic_long_read(&rsp->expedited_start),
 			 (ulong)atomic_long_read(&rsp->expedited_done) +
 			 ULONG_MAX / 8)) {
-		synchronize_sched();
+		wait_rcu_gp(call_rcu_sched);
 		atomic_long_inc(&rsp->expedited_wrap);
 		return;
 	}