hexdump: add FIEMAP-based sparse file optimization

hexdump: add FIEMAP-based sparse file optimization

Use Linux FIEMAP ioctl to detect sparse file holes and skip reading
them. This significantly improves performance when dumping large
sparse files by avoiding unnecessary disk I/O for hole regions.

Key changes:
- Add fiemap, in_sparse_hole, and region_end fields to struct hexdump
- Use check_hole() with cached region boundaries to minimize syscalls
- Only skip holes when vflag==DUP to preserve correct "*" output
- Distinguish real sparse holes from duplicate data in regular files