net: add common accessor for setting dropcount on packets

As part of an effort to move skb->dropcount to skb->cb[], use
a common function in order to set dropcount in struct sk_buff.

Signed-off-by: Eyal Birger <eyal.birger@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc include/net/sock.h
Simple merge

diff --cc net/core/sock.c
Simple merge

diff --cc net/packet/af_packet.c
Simple merge