logalloc: reduce gap between std min_free and logalloc min_free
logalloc: reduce gap between std min_free and logalloc min_free

With the larger gap, logalloc reserved more memory for std than
the background reclaim threshold for running, so it was triggered
rarely.

With the gap reduced, background reclaim is constantly running in
an allocating workload (e.g. cache misses).

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