skb: add some helpers for skb drop reasons

In order to simply the definition and assignment for
'enum skb_drop_reason', introduce some helpers.

SKB_DR() is used to define a variable of type 'enum skb_drop_reason'
with the 'SKB_DROP_REASON_NOT_SPECIFIED' initial value.

SKB_DR_SET() is used to set the value of the variable. Seems it is
a little useless? But it makes the code shorter.

SKB_DR_OR() is used to set the value of the variable if it is not set
yet, which means its value is SKB_DROP_REASON_NOT_SPECIFIED.

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>

include/linux/skbuff.h

@@ -450,6 +450,18 @@ enum skb_drop_reason {
 	SKB_DROP_REASON_MAX,
 };
 
+#define SKB_DR_INIT(name, reason)				\
+	enum skb_drop_reason name = SKB_DROP_REASON_##reason
+#define SKB_DR(name)						\
+	SKB_DR_INIT(name, NOT_SPECIFIED)
+#define SKB_DR_SET(name, reason)				\
+	(name = SKB_DROP_REASON_##reason)
+#define SKB_DR_OR(name, reason)					\
+	do {							\
+		if (name == SKB_DROP_REASON_NOT_SPECIFIED)	\
+			SKB_DR_SET(name, reason);		\
+	} while (0)
+
 /* To allow 64K frame to be packed as single skb without frag_list we
  * require 64K/PAGE_SIZE pages plus 1 additional page to allow for
  * buffers which do not start on a page boundary.