reduce allocations on multi-disk clusters (#12311)

multi-disk clusters initialize buffer pools
per disk, this is perhaps expensive and perhaps
not useful, for a running server instance. As this
may disallow re-use of buffers across sets,

this change ensures that buffers across sets
can be re-used at drive level, this can reduce
quite a lot of memory on large drive setups.

cmd/erasure-healing-common.go

@@ -261,7 +261,7 @@ func disksWithAllParts(ctx context.Context, onlineDisks []StorageAPI, partsMetad
 		// Always check data, if we got it.
 		if (len(meta.Data) > 0 || meta.Size == 0) && len(meta.Parts) > 0 {
 			checksumInfo := meta.Erasure.GetChecksumInfo(meta.Parts[0].Number)
-			dataErrs[i] = bitrotVerify(bytes.NewBuffer(meta.Data),
+			dataErrs[i] = bitrotVerify(bytes.NewReader(meta.Data),
 				int64(len(meta.Data)),
 				meta.Erasure.ShardFileSize(meta.Size),
 				checksumInfo.Algorithm,