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).

utils/logalloc.cc

@@ -1012,7 +1012,7 @@ void segment_pool::prime(size_t available_memory, size_t min_free_memory) {
     // We want to leave more free memory than just min_free_memory() in order to reduce
     // the frequency of expensive segment-migrating reclaim() called by the seastar allocator.
     size_t min_gap = 1 * 1024 * 1024;
-    size_t max_gap = 64 * 1024 * 1024;
+    size_t max_gap = 32 * 1024 * 1024;
     size_t gap = std::min(max_gap, std::max(available_memory / 16, min_gap));
     _store.non_lsa_reserve = min_free_memory + gap;
     // Since the reclaimer is not yet in place, free some low memory for general use