ipv6: sr: clear IP6CB(skb) on SRH ip4ip6 encapsulation

skb->cb may contain data from previous layers (in an observed case
IPv4 with L3 Master Device). In the observed scenario, the data in
IPCB(skb)->frags was misinterpreted as IP6CB(skb)->frag_max_size,
eventually caused an unexpected IPv6 fragmentation in ip6_fragment()
through ip6_finish_output().

This patch clears IP6CB(skb), which potentially contains garbage data,
on the SRH ip4ip6 encapsulation.

Fixes: 32d99d0b ("ipv6: sr: add support for ip4ip6 encapsulation")
Signed-off-by: Yohei Kanemaru <yohei.kanemaru@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>