net: icmpv6: Store ll addresses to pkt when sending error msg

When we are sending ICMPv6 error message, we need to store the
link local addresses of the received packet somewhere in order
to know where to send the new error message.
Easiest is to store the ll addresses is to the error message
itself, just before where the sent packet will start in memory.
We cannot use the original pkt to store the ll addresses
as that packet might get overwritten if we receive lot of packets.

Fixes #29398

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