get_clones_stat() could be very slow if a snapshot has many (thousands) clones.
Clone names are added to an nvlist that's created with NV_UNIQUE_NAME.
So, each time a new name is appended to the list, the whole list is searched
linearly to see if that name is not already in the list. That results in the
quadratic complexity.
That should be easy to fix as we know in advance that we should not get any
duplicate names, so we can drop NV_UNIQUE_NAME when creating the list.