XL: Make allocations simpler avoid redundant allocs. (#1961)

- Reduce 10MiB buffers for loopy calls to use 128KiB.
- start using 128KiB buffer where needed.

posix.go

@@ -418,14 +418,9 @@ func (s *posix) ReadFile(volume string, path string, offset int64, buf []byte) (
 	// Close the reader.
 	defer file.Close()
 
-	// Read file.
+	// Read full until buffer.
 	m, err := io.ReadFull(file, buf)
 
-	// Error unexpected is valid, set this back to nil.
-	if err == io.ErrUnexpectedEOF {
-		err = nil
-	}
-
 	// Success.
 	return int64(m), err
 }