This functor use seeks operation to perform fast sparse copy between blocks.
Copy all mapped data from a source SEEKABLE device to a destination BLOCK device.
This function will preserve sparseness information in the source disk. The destination block device must be pre-zeroed, otherwise previous data will "leak through".
Fails with `Different_sizes if the source and destination are not exactly the same size.
Fails with `Is_read_only if the destination device is read-only.