Bluetooth: Use bt_cb(skb)->chan to send raw data back

Instead of accessing skb->sk in L2CAP core we now compare the channel
a skb belongs to and not send it back if the channel is same. This change
removes another struct socket usage from L2CAP core.

Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>