table: get_snapshot_details: count allocated_size
table: get_snapshot_details: count allocated_size

Rather than the logical file sizes so to account
for metadata overhead.

Fixes #9898

Signed-off-by: Benny Halevy <23c997c90eb8537635e9392d48e7af1bf1b2ca22@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla