Add ioctl FIEMAP #1639

I want to continue using copy_file_range if and only if 1st 64 KiB was reflink'ed for performantical reason. The usecase is covered by using FIEMAP for copied bytes. There is FICLONERANGE, but it is difficult to use due to alignment.

There is already existing safe wrapper: https://github.com/lilydjwg/fiemap-rs .