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>