MB-7938: Don't decr diskQueueSize if an item is being requeued

Change-Id: Iec7270e577ed8faae18078c5eed6dfbf89e795ac
Reviewed-on: http://review.couchbase.org/25880
Reviewed-by: Chiyoung Seo <chiyoung.seo@gmail.com>
Tested-by: Michael Wiederhold <mike@couchbase.com>