app/testpmd: add MPLSoGRE encapsulation

app/testpmd: add MPLSoGRE encapsulation

Example for MPLSoGRE tunnel:
ETH / IPV4 / GRE / MPLS / IP / L4..L7

In order to encapsulate such a tunnel there is a need to remove L2 of
the inner packet and encap the remaining tunnel, this is done by
applying 2 rte flow commands l2_decap followed by mplsogre_encap.
Both commands must appear in the same flow, and from the point of the
packet it both actions are applied at the same time. (There is no part
where a packet doesn't have L2 header).

Decapsulating such a tunnel works the other way, first we need to decap
the outer tunnel header and then apply the new L2.
So the commands will be mplsogre_decap / l2_encap

Due to the complex encapsulation of MPLSoGRE flow action and
based on the fact testpmd does not allocate memory, this patch adds a
new command in testpmd to initialise a global structure containing the
necessary information to make the outer layer of the packet.  This same
global structures will then be used by the flow commands in testpmd when
the action mplsogre_encap, mplsogre_decap, will be parsed, at this
point, the conversion into such action becomes trivial.

Signed-off-by: Ori Kam <orika@mellanox.com>
Reviewed-by: Ferruh Yigit <ferruh.yigit@intel.com>