nwfilter: Fix learning address thread shutdown

nwfilter: Fix learning address thread shutdown

If the learning thread is configured to learn on all ethernet frames
(which is hardcoded) then chances are high that there is a packet on
every iteration of inspecting frames loop. As result we will hang on
shutdown because we don't check threadsTerminate if there is packet.

Let's just check termination conditions on every iteration. Since
we'll check each iteration, the check after pcap_next essentially
is unnecessary since on failure we'd loop back to the top and timeout
and then fail.

Signed-off-by: Nikolay Shirokovskiy <nshirokovskiy@virtuozzo.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>