sstables: accurate summary entry size calculation
sstables: accurate summary entry size calculation

Calculate the summary entry size correctly, so we don't end up with oversize
summaries.
Message-Id: <a06611cd84629926e2d116fbbfa512d5d71538c9@scylladb.com>

duarten/scylla,duarten/scylla,scylladb/scylla,scylladb/scylla,duarten/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla