btrfs: test send optimal cloning behaviour

btrfs: test send optimal cloning behaviour

Test that send operations do the best cloning decisions when we have
extents that are shared but some files refer to the full extent while
others refer to only a section of the extent.

This exercises an optimization that was added to kernel 6.2, by the
following commit:

  c7499a64dcf6 ("btrfs: send: optimize clone detection to increase extent sharing")

Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: David Disseldorp <ddiss@suse.de>
Signed-off-by: Zorro Lang <zlang@kernel.org>