gre_offload: statically build GRE offloading support

GRO/GSO layers can be enabled on a node, even if said
node is only forwarding packets.

This patch permits GSO (and upcoming GRO) support for GRE
encapsulated packets, even if the host has no GRE tunnel setup.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: H.K. Jerry Chu <hkchu@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --cc include/net/gre.h
Simple merge

diff --cc net/ipv4/Makefile
Simple merge

diff --cc net/ipv4/gre_demux.c
Simple merge

diff --cc net/ipv4/gre_offload.c
Simple merge