fix warning of unused endSeqno in BackfillDiskLoad

Change-Id: I678a38e3551d0260f5e7664675a3bd2fd0573726
Reviewed-on: http://review.couchbase.org/37661
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Sundararaman Sridharan <sundar@couchbase.com>