vlan: make a new function vlan_dev_vlan_proto() and export

The vlan support 2 proto: 802.1q and 802.1ad, so make a new function
called vlan_dev_vlan_proto() which could return the vlan proto for
input dev.

Signed-off-by: Ding Tianhong <dingtianhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>