net: gre: move GSO functions to gre_offload

Similarly to TCP/UDP offloading, move all related GRE functions to
gre_offload.c to make things more explicit and similar to the rest
of the code.

Suggested-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
diff --git a/include/net/gre.h b/include/net/gre.h
index a5a4ddf..57e4afd 100644
--- a/include/net/gre.h
+++ b/include/net/gre.h
@@ -32,6 +32,10 @@
 
 int gre_cisco_register(struct gre_cisco_protocol *proto);
 int gre_cisco_unregister(struct gre_cisco_protocol *proto);
+
+int gre_offload_init(void);
+void gre_offload_exit(void);
+
 void gre_build_header(struct sk_buff *skb, const struct tnl_ptk_info *tpi,
 		      int hdr_len);
 struct sk_buff *gre_handle_offloads(struct sk_buff *skb, bool gre_csum);