Add clear error for kernel not supporting IPv6

Make it clear to the user, that if adding a tunnel fails with kernel GTP
and IPv6: the reason is that the kernel doesn't support IPv6 yet.

Related: OS#6096
Change-Id: I1d3c8cbb51212c91136292347dad9529a5c58a31
---
M ggsn/ggsn.c
1 file changed, 15 insertions(+), 0 deletions(-)

diff --git a/ggsn/ggsn.c b/ggsn/ggsn.c
index 0cd0feb..0603773 100644
--- a/ggsn/ggsn.c
+++ b/ggsn/ggsn.c
@@ -534,6 +534,8 @@
 	if (apn->cfg.gtpu_mode == APN_GTPU_MODE_KERNEL_GTP) {
 		if (gtp_kernel_tunnel_add(pdp, apn->tun.cfg.dev_name) < 0) {
 			LOGPPDP(LOGL_ERROR, pdp, "Cannot add tunnel to kernel: %s\n", strerror(errno));
+			if (addrv6 && errno == EINVAL)
+				LOGPPDP(LOGL_ERROR, pdp, "Maybe your kernel does not support GTP-U with IPv6 yet?\n");
 			gtp_create_context_resp(gsn, pdp, GTPCAUSE_SYS_FAIL);
 			return 0;
 		}

