net: Add onlink and forwarding check to IPv6-prepare

Deadloop happens when CONFIG_NET_ROUTING and VLAN are enabled.
In function net_ipv6_prepare_for_send(), pkt->iface will be
updated with net_pkt_set_iface(pkt, iface) in 2 scenarios:
1. ip_hdr->dst is onlink
2. check_route or nbr_lookup
VLAN is virtual-iface which attaches to a physical-iface. Each
time a packet being sent to a VLAN port will invoke twice of
the net_send_data(). The 1st time, pkt->iface is set to virtual
iface and the 2nd time to physical iface.
However in above 2 scenarios, at the 2nd time of calling the
net_send_data(), the pkt-iface will be changed back to virtual
iface. The system runs into a deadloop. This can be proved by
enabling CONFIG_NET_ROUTING with the VLAN sample.
The main purpose for net_ipv6_prepare_for_send() is to set the
right ll_dst address. If the ll_dst address is already set, then
no need to go through it again. If the packet has done with the
forwarding and set the ll_dst, then no need to check_route again.
And, the pkt->iface will not be changed back to virtual iface.

(cherry picked from commit 0b24b96)

Original-Fixes: #77402
Original-Signed-off-by: Shrek Wang <shrek.wang@nxp.com>
GitOrigin-RevId: 0b24b96
Cr-Build-Id: 8736464299640945217
Cr-Build-Url: https://cr-buildbucket.appspot.com/build/8736464299640945217
Copybot-Job-Name: zephyr-main-copybot-downstream
Change-Id: I908768dc7b747e49b4e51ab2568a458945646b52
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/zephyr/+/5873058
Commit-Queue: Tristan Honscheid <honscheid@google.com>
Tested-by: Tristan Honscheid <honscheid@google.com>
Tested-by: ChromeOS Prod (Robot) <chromeos-ci-prod@chromeos-bot.iam.gserviceaccount.com>
Reviewed-by: Tristan Honscheid <honscheid@google.com>

subsys/net/ip/ipv6_nbr.c

@@ -852,7 +852,9 @@ enum net_verdict net_ipv6_prepare_for_send(struct net_pkt *pkt)
 	 */
 	if ((net_pkt_lladdr_dst(pkt)->addr &&
 	     ((IS_ENABLED(CONFIG_NET_ROUTING) &&
-	      net_ipv6_is_ll_addr((struct in6_addr *)ip_hdr->dst)) ||
+	      (net_ipv6_is_ll_addr((struct in6_addr *)ip_hdr->dst) ||
+	       net_if_ipv6_addr_onlink(NULL, (struct in6_addr *)ip_hdr->dst) ||
+	       net_pkt_forwarding(pkt))) ||
 	      !IS_ENABLED(CONFIG_NET_ROUTING))) ||
 	    net_ipv6_is_addr_mcast((struct in6_addr *)ip_hdr->dst) ||
 	    /* Workaround Linux bug, see: