net: tcp2: Fix sending to 6lo based networks

Do not send the original pkt in 6lo based networks as in those
the IPv6 header is mangled and we would not be able to do any
resends of the original pkt. So for 6lo networks, clone the
pkt and send it to peer. The original pkt is kept in sent list
in case we need to resend to peer.

Fixes #29771

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