tun: not use hardcoded mask value

0x3ff in tun_hashfn is mask of TUN_NUM_FLOW_ENTRIES, instead
of hardcode, define a macro to setup the relationship with
TUN_NUM_FLOW_ENTRIES
Signed-off-by: Li RongQing <lirongqing@baidu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/tun.c

@@ -200,6 +200,7 @@ struct tun_flow_entry {
 };
 
 #define TUN_NUM_FLOW_ENTRIES 1024
+#define TUN_MASK_FLOW_ENTRIES (TUN_NUM_FLOW_ENTRIES - 1)
 
 struct tun_prog {
 	struct rcu_head rcu;
@@ -406,7 +407,7 @@ static inline __virtio16 cpu_to_tun16(struct tun_struct *tun, u16 val)
 
 static inline u32 tun_hashfn(u32 rxhash)
 {
-	return rxhash & 0x3ff;
+	return rxhash & TUN_MASK_FLOW_ENTRIES;
 }
 
 static struct tun_flow_entry *tun_flow_find(struct hlist_head *head, u32 rxhash)