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.

Fixes: #77402

Signed-off-by: Shrek Wang <shrek.wang@nxp.com>