ZFS_IOC_SEND_PROGRESS requires the name of the snapshot currently being transferred

[asomers]

The ZFS_IOC_SEND_PROGRESS ioctl requires the name of the snapshot currently being transferred. py-libzfs actually passes the name of the snapshot that was used to initiate the transfer. That works when sending a single snapshot, but fails when sending multiple snapshots (such as with zfs send's -I, -i, and -R options).
I have a patch for this. However, it still doesn't work to answer the question "how far along is this send that includes multiple snapshots?" because the calculated progress can exceed 100%. I think that's a limitation in libzfs, not py-libzfs. I've since abandoned ZFS_IOC_SEND_PROGRESS entirely so I'm not going to work on that patch anymore. Let me know if you would be interested in the partial solution.