gtp-genl: allocate room for maximum IPv6 address

Otherwise, inet_ntop() might return ENOSPC because of the buffer
being too small to accomodate an IPv6 address.

Change-Id: I2283e0c3112bec8e6e7e7b5c96657facc09d0a7a
diff --git a/src/gtp-genl.c b/src/gtp-genl.c
index b8edc72..9c9e8d3 100644
--- a/src/gtp-genl.c
+++ b/src/gtp-genl.c
@@ -155,7 +155,7 @@
 static int genl_gtp_attr_cb(const struct nlmsghdr *nlh, void *data)
 {
 	struct nlattr *tb[GTPA_MAX + 1] = {};
-	char buf[INET_ADDRSTRLEN];
+	char buf[INET6_ADDRSTRLEN];
 	struct gtp_pdp pdp = {};
 	struct genlmsghdr *genl;