vlan: rename __vlan_put_tag to vlan_insert_tag_set_proto

Name fits better. Plus there's going to be introduced
__vlan_insert_tag later on.

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Acked-by: Pravin B Shelar <pshelar@nicira.com>
Signed-off-by: David S. Miller <davem@davemloft.net>