[VLAN]: Move protocol determination to seperate function

I think, that we can make this code flow easier to understand
by introducing the vlan_set_encap_proto() function (I hope the
name is good) to setup the skb proto and merge the paths calling
netif_rx() together.

[Patrick: Modified to apply on top of my previous patches]

Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>