Remove compiler warning from comparison

warning: comparison between signed and unsigned integer expressions [-Wsign-compare]
     if (blockSize < seek_result) {

Change-Id: Ia014128a362cc192401c411e926e2661071ea178
Reviewed-on: http://review.couchbase.org/37168
Reviewed-by: Chiyoung Seo <chiyoung@couchbase.com>
Tested-by: Chiyoung Seo <chiyoung@couchbase.com>