kvevent: Avoid busy loop during out of quota

Previous PR cockroachdb#87464 erroneously removed code to ensure
that consumer is notified about out of quota events
once.  Rectify this issue.

Release Justification: bug fix
Release note: None

pkg/ccl/changefeedccl/kvevent/blocking_buffer.go

@@ -132,6 +132,9 @@ func (b *blockingBuffer) pop() (e Event, ok bool, err error) {
 		// So, we issue the flush request to the consumer to ensure that we release some memory.
 		e = Event{flush: true}
 		ok = true
+		// Ensure we notify only once.  If we're still out of quota,
+		// subsequent notifyOutOfQuota will reset this field.
+		b.mu.canFlush = false
 	}
 
 	if b.mu.drainCh != nil && b.mu.queue.empty() {