net/sched: fix miss init the mru in qdisc_skb_cb

The mru in the qdisc_skb_cb should be init as 0. Only defrag packets in the
act_ct will set the value.

Fixes: 038ebb1a ("net/sched: act_ct: fix miss set mru for ovs after defrag in act_ct")
Signed-off-by: wenxu <wenxu@ucloud.cn>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>