net: netfilter: use kfree_drop_reason() for NF_DROP

Replace kfree_skb() with kfree_skb_reason() in nf_hook_slow() when
skb is dropped by reason of NF_DROP. Following new drop reasons
are introduced:

SKB_DROP_REASON_NETFILTER_DROP

Signed-off-by: Menglong Dong <imagedong@tencent.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/skbuff.h

@@ -320,6 +320,7 @@ enum skb_drop_reason {
 	SKB_DROP_REASON_TCP_CSUM,	/* TCP checksum error */
 	SKB_DROP_REASON_SOCKET_FILTER,	/* dropped by socket filter */
 	SKB_DROP_REASON_UDP_CSUM,	/* UDP checksum error */
+	SKB_DROP_REASON_NETFILTER_DROP,	/* dropped by netfilter */
 	SKB_DROP_REASON_MAX,
 };