Add generic IP packet code and use it for GRE.

Bug: 11542311
Change-Id: I174e0b268869bc77927feeee57003580c47c30f2
diff --git a/translate.h b/translate.h
index c4d8ede..9f1ac15 100644
--- a/translate.h
+++ b/translate.h
@@ -57,6 +57,9 @@
 int icmp6_to_icmp(clat_packet out, int pos, const struct icmp6_hdr *icmp6,
                   const char *payload, size_t payload_size);
 
+// Translate generic IP packets.
+int generic_packet(clat_packet out, int pos, const char *payload, size_t len);
+
 // Translate TCP and UDP packets.
 int tcp_packet(clat_packet out, int pos, const struct tcphdr *tcp, uint32_t checksum, size_t len);
 int udp_packet(clat_packet out, int pos, const struct udphdr *udp, uint32_t checksum, size_t len);