sk_chk_filter — verify socket filter code
int sk_chk_filter (struct sock_filter * filter, int flen);
Check the user's filter code. If we let some ugly filter code slip through kaboom! The filter must contain no references or jumps that are out of range, no illegal instructions and no backward jumps. It must end with a RET instruction
Returns 0 if the rule set is legal or a negative errno code if not.