net: skb: change the definition SKB_DR_SET()

The SKB_DR_OR() is used to set the drop reason to a value when it is
not set or specified yet. SKB_NOT_DROPPED_YET should also be considered
as not set.

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