Bluetooth: Assign L2CAP socket priority when allocating SKB

The SKB for L2CAP sockets are all allocated in a central callback
in the socket support. Instead of having to pass around the socket
priority all the time, assign it to skb->priority when actually
allocating the SKB.

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