Add IPv6 support

Implement IPv6 in libgtpnl and the gtp-tunnel testing tool. Allow to
combine:

- GTPA_MS_ADDRESS and GTPA_PEER_ADDR6
- GTPA_MS_ADDR6 and GTPA_PEER_ADDRESS

to specify IPv4-in-IPv6-GTP and IPv6-in-IPv4-GTP in the tunnel
declaration from control plane.

This patch is based on multiple patches from Pablo in OS#6123. I decided
to squash them to directly implement v4-in-v6 and vice versa, instead of
implementing another variant first and then changing it again.

Co-developed-by: Pablo Neira Ayuso <pablo@netfilter.org>
Related: OS#6096
Change-Id: If864c9170f74af52a95cbc4cdb1b866e0309306b
diff --git a/include/linux/gtp.h b/include/linux/gtp.h
index 3dcdb9e..40f5388 100644
--- a/include/linux/gtp.h
+++ b/include/linux/gtp.h
@@ -31,6 +31,9 @@
 	GTPA_I_TEI,	/* for GTPv1 only */
 	GTPA_O_TEI,	/* for GTPv1 only */
 	GTPA_PAD,
+	GTPA_PEER_ADDR6,
+	GTPA_MS_ADDR6,
+	GTPA_FAMILY,
 	__GTPA_MAX,
 };
 #define GTPA_MAX (__GTPA_MAX - 1)