is_seen
int (*is_seen)(struct ctl_table_set *))
set->is_seen = is_seen;
else if (!set->is_seen)
res = set->is_seen(set);
int (*is_seen)(struct ctl_table_set *);
int (*is_seen)(struct ctl_table_set *));
int (*is_seen)(struct ctl_table_set *))
setup_sysctl_set(&net->sysctls, &net_sysctl_root, is_seen);