af-packet: speed up thread sync during startup

[victorjulien]

Threads are initialized sequentially to allow for a predictable mapping of threads and queues. Not all parts of the start up need to be done sequentially. The setting up of the rings can be very expensive, taking of a couple of hundred milliseconds. The ring setup doesn't need to be done sequentially though.

This patch releases the thread early, after bind but before the ring setups.