Fix tc stats when using -batch mode

There are two global variables in tc/tc_class.c:
__u32 filter_qdisc;
__u32 filter_classid;

These are not re-initialized for each line received in -batch mode:
# tc -force -batch -
class show dev eth0 parent 1: classid 1:1
class show dev eth0 parent 1: classid 1:1
Error: duplicate "classid": "1:1" is the second value.

This patch fixes the issue by initializing the two globals when we
enter print_class().