torture/ioctl: add FSCTL_DUP_EXTENTS_TO_FILE tests

FSCTL_DUPLICATE_EXTENTS_TO_FILE is yet another copy offload mechanism,
this time only targeting COW FSes, where the request triggers a meta-
data only clone of the source range.

These tests attempt to cover most of the normal use cases, as well as
number of more exotic scenarios.

FILE_SUPPORTS_BLOCK_REFCOUNTING FS attribute presence is checked prior
to running the tests, so they will currently be skipped during Samba
self test (which lacks the flag).

Signed-off-by: David Disseldorp <[email protected]>
Reviewed-by: Jeremy Allison <[email protected]>