datapath: VLAN actions should use push/pop semantics

datapath: VLAN actions should use push/pop semantics

Currently the kernel vlan actions mirror those used by OpenFlow 1.0.
i.e. MODIFY and STRIP. More flexible approach is to have an action to
push a tag and pop a tag off, so that it can handle multiple levels of vlan
tags. Plus it aligns with newer version of OpenFlow.
        As this patch replaces MODIFY with PUSH semantic, action
mapping done in userpace is fixed accordingly.
        GSO handling for multiple levels of vlan tags is also added as
Jesse suggested before.

Signed-off-by: Pravin B Shelar <pshelar@nicira.com>
Acked-by: Jesse Gross <jesse@nicira.com>