allow disabling O_DIRECT in certain environments for reads (#14115)

repeated reads on single large objects in HPC like
workloads, need the following option to disable
O_DIRECT for a more effective usage of the kernel
page-cache.

However this optional should be used in very specific
situations only, and shouldn't be enabled on all
servers.

NVMe servers benefit always from keeping O_DIRECT on.

cmd/xl-storage.go

@@ -1652,7 +1652,7 @@ func (s *xlStorage) ReadFileStream(ctx context.Context, volume, path string, off
 	}
 
 	alignment := offset%xioutil.DirectioAlignSize == 0
-	if !alignment {
+	if !alignment || globalAPIConfig.isDisableODirect() {
 		if err = disk.DisableDirectIO(file); err != nil {
 			file.Close()
 			return nil, err