datapath: move vlan pop/push functions into common code

datapath: move vlan pop/push functions into common code

So it can be used from out of openvswitch code.
Did couple of cosmetic changes on the way, namely variable naming and
adding support for 8021AD proto.

Note on backwards compatability:
Unlike the upstream version, the backport of skb_vlan_push() does not
support translating a hardware accelerated 8021AD tag to software.
This is not a problem though as it preserves existing behaviour.

Upstream: 93515d53 ("net: move vlan pop/push functions into common code")
Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>