Sending "empty" snapshots allows inserting snapshots out of order

[manfromafar]

System information

Type	Version/Name
Distribution Name	Ubuntu
Distribution Version	24.04.1
Kernel Version	6.8.0-48-generic
Architecture	amd64
OpenZFS Version	zfs-2.2.6-1

Describe the problem you're observing

When creating empty snapshots with 0 used space you can send snapshots out of order.
For example if you can send snapshots 1 and 3 to a new dataset.
Once the send is complete you can then send snapshot 2 to the new dataset and it'll be inserted without issue by zfs.

Unfortunately this only works if the snapshots are empty once actual data is written zfs with throw a error about changes on the remote dataset.

Describe how to reproduce the problem

1. Create a dataset lets called test on a pool called tank
2. snapshot tank/test multiple times without making changes
   a. for x in {1..3}; do zfs snapshot tank/test@$x; done
3. Send an initial replication of snapshot 1 from tank/test to tank/test2
   a. zfs send tank/test@1 | zfs recv tank/test2
4. Send a incremental of snapshots 1 and 3 from tank/test to tank/test2.
   a. zfs send -i tank/test@1 tank/test@3 |zfs recv tank/test2
5. On tank/test2 we should see snapshots 1 and 3
   a. zfs list -t snapshot tank/test2
tank/test2@1 0B - 24K - tank/test2@3 0B - 24K -
   b. Both snapshots are 0 used as nothing has changed between them.
6. Now do another incremental send of snapshots 1 and 2 from tank/test
   a. zfs send -I tank/test@1 tank/test@2 |zfs recv tank/test2
7. We should now see that snapshots 1,2,3 all exist for tank/test2 even though they were sent out of order. Which would normally require a rollback.
   a. zfs list -t snapshot tank/test2
tank/test2@1 0B - 24K - tank/test2@3 0B - 24K - tank/test2@2 0B - 24K -
8. Even though the snapshots are empty we've effectively backfilled a snapshot.

Include any warning/errors/backtraces from the system logs