vlan: introduce __vlan_insert_tag helper which does not free skb

There's a need for helper which inserts vlan tag but does not free the
skb in case of an error.

Suggested-by: Pravin Shelar <pshelar@nicira.com>
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>