Minor fix for the `CompactMap()` performance test. (#6836)

Per-entry memory usage is based on `TotalAllocs`, which is incorrect - that
value is a cummulative of heap usage, which doesn't decrease when objects
are freeed.

`Allocs` is instead an accurate represeentation of actual memory usage
at the time metrics are reported.

weed/storage/needle_map/compact_map_perf_test.go

@@ -16,7 +16,7 @@ import (
 To see the memory usage:
 
 go test -run TestMemoryUsage
-The TotalAlloc section shows the memory increase for each iteration.
+The Alloc section shows the in-use memory increase for each iteration.
 
 go test -run TestMemoryUsage -memprofile=mem.out
 go tool pprof --alloc_space needle.test mem.out
@@ -81,7 +81,7 @@ func PrintMemUsage(totalRowCount uint64) {
 	var m runtime.MemStats
 	runtime.ReadMemStats(&m)
 	// For info on each, see: https://golang.org/pkg/runtime/#MemStats
-	fmt.Printf("Each %.2f Bytes", float64(m.TotalAlloc)/float64(totalRowCount))
+	fmt.Printf("Each %.02f Bytes", float64(m.Alloc)/float64(totalRowCount))
 	fmt.Printf("\tAlloc = %v MiB", bToMb(m.Alloc))
 	fmt.Printf("\tTotalAlloc = %v MiB", bToMb(m.TotalAlloc))
 	fmt.Printf("\tSys = %v MiB", bToMb(m.Sys))