net: ipv6: add skb drop reasons to ip6_protocol_deliver_rcu()

Replace kfree_skb() used in ip6_protocol_deliver_rcu() with
kfree_skb_reason().

No new reasons are added.

Some paths are ignored, as they are not common, such as encapsulation
on non-final protocol.

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>