* zfs: Fix handling of parameters passed via check_invalid_arguments
cc7a5228 had a typo, so the merged set of arguments was shoved into the
wrong parameter and ignored.
`origin` is an actual module parameter and should be processed like one.
pop()ing makes debug output misleading.
* zfs: fix command generation for `zfs snapshot`
Creating a snapshot and supplying an origin are mutually exclusive,
but were not treated as such. We should throw an error instead of
running an invalid command (`zfs snapshot origin foo@snapname`.)