logalloc: background reclaim: use default scheduling group for adjusting shares
logalloc: background reclaim: use default scheduling group for adjusting shares

If the shares are currently low, we might not get enough CPU time to
adjust the shares in time.

This is currently no-op, since Seastar runs the callback outside
scheduling groups (and only uses the scheduling group for inherited
continuations); but better be insulated against such details.

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