Bluetooth: Use explicit header and body length for L2CAP SKB allocation

When allocating the L2CAP SKB for transmission, provide the upper layers
with a clear distinction on what is the header and what is the body
portion of the SKB.

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