net: ipv6: add skb drop reasons to ip6_pkt_drop()

Replace kfree_skb() used in ip6_pkt_drop() with kfree_skb_reason().
No new reason is added.

Signed-off-by: Menglong Dong <imagedong@tencent.com>
Reviewed-by: Jiang Biao <benbjiang@tencent.com>
Reviewed-by: Hao Peng <flyingpeng@tencent.com>
Signed-off-by: David S. Miller <davem@davemloft.net>