Bluetooth: Update L2CAP socket source address from HCI connection

When having LE connections, the source address is not always the
public address of the controller. So update the socket address
based on the actual used source address of the HCI connection.

This also remove the pointless source address pointer and adds
a proper lock around the socket structure.

Signed-off-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>