Expose the new extent-same ioctl.
Add a high-level interface to the same-extent ioctl.

This interface deals with larger ranges
by repeatedly calling the ioctl.
Use the dedup ioctl to deduplicate.
Align on 4k.
(syscall wip) No need for immutability-locking
(syscall wip) No need to skip frozen snapshots
(syscall wip) No need to hash
Dedup len note

Exposing the 16M limit in the abi is icky

Signed-off-by: Gabriel <g2p.code@gmail.com>
Cope with large dedup clusters (128+)