gro: support VxLAN GRO

gro: support VxLAN GRO

This patch adds a framework that allows GRO on tunneled packets.
Furthermore, it leverages that framework to provide GRO support for
VxLAN-encapsulated packets. Supported VxLAN packets must have an outer
IPv4 header, and contain an inner TCP/IPv4 packet.

VxLAN GRO doesn't check if input packets have correct checksums and
doesn't update checksums for output packets. Additionally, it assumes
the packets are complete (i.e., MF==0 && frag_off==0), when IP
fragmentation is possible (i.e., DF==0).

Signed-off-by: Jiayu Hu <jiayu.hu@intel.com>
Reviewed-by: Junjie Chen <junjie.j.chen@intel.com>
Tested-by: Lei Yao <lei.a.yao@intel.com>