datapath: Add __vlan_insert_tag() compat helper if not available

datapath: Add __vlan_insert_tag() compat helper if not available

Since older kernels do not have skb->vlan_proto, it is assumed that
kernels which don't provide their own __vlan_insert_tag() will also
not have skb->vlan_proto. The backwards compat function therefore
only supports ETH_P_8021Q as the protocol type.

Upstream: 15255a43 ("vlan: introduce __vlan_insert_tag helper which does not free skb")
Signed-off-by: Thomas Graf <tgraf@noironetworks.com>
Acked-by: Pravin B Shelar <pshelar@nicira.com>