commitlog: Include segment wasted (slack) size in footprint check
commitlog: Include segment wasted (slack) size in footprint check

Refs #8270

Since segment allocation looks at actual disk footprint, not active,
the threshold check in timer task should include slack space so we
don't mistake sparse usage for space left.

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