net: ipv6: add skb drop reasons to ip6_rcv_core()

Replace kfree_skb() used in ip6_rcv_core() with kfree_skb_reason().
No new drop reasons are added.

Seems now we use 'SKB_DROP_REASON_IP_INHDR' for too many case during
ipv6 header parse or check, just like what 'IPSTATS_MIB_INHDRERRORS'
do. Will it be too general and hard to know what happened?

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>