These pernet_operations initialize and destroy
net_generic(net, ip_set_net_id)-related data.
Since ip_set is under CONFIG_IP_SET, it's easy
to watch drivers, which depend on this config.
All of them are in net/netfilter/ipset directory,
except of net/netfilter/xt_set.c. There are no
more drivers, which use ip_set, and all of
the above don't register another pernet_operations.
Also, there are is no indirect users, as header
file include/linux/netfilter/ipset/ip_set.h does
not define indirect users by something like this: