af-packet: speed up thread sync during startup

af-packet: speed up thread sync during startup

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.

Ticket: #7272.