net: pkt: Allow larger packets for IPv6 fragmentation

If the allocated net_pkt is larger than the network interface MTU,
then check if the IPv6 fragmentation is enabled and allow larger
net_pkt length as the IPv6 fragmentation will split the packet into
suitable parts.

Fixes #16354

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>

subsys/net/ip/net_pkt.c

@@ -954,6 +954,13 @@ static size_t pkt_buffer_length(struct net_pkt *pkt,
 
 	/* Family vs iface MTU */
 	if (IS_ENABLED(CONFIG_NET_IPV6) && family == AF_INET6) {
+		if (IS_ENABLED(CONFIG_NET_IPV6_FRAGMENT) && (size > max_len)) {
+			/* We support larger packets if IPv6 fragmentation is
+			 * enabled.
+			 */
+			max_len = size;
+		}
+
 		max_len = MAX(max_len, NET_IPV6_MTU);
 	} else if (IS_ENABLED(CONFIG_NET_IPV4) && family == AF_INET) {
 		max_len = MAX(max_len, NET_IPV4_MTU);