nwfilter: Use immediate packet delivery mode rather than buffering

nwfilter: Use immediate packet delivery mode rather than buffering

Our nwfilter code doesn't set any timeout on the pcap packet buffer which
means that when DHCP snooping is enabled on a guest interface and
libvirt is trying to learn the IP address from guest's DHCP traffic, it
takes up to 4x longer to ping a guest successfully compared to a case
where nwfilter isn't enabled at all or libvirt uses the cached nwfilter
leases to populate the corresponding rules to ebtables.
With the pcap filter and rate limiting already in place, we should be
able to afford enabling the immediate packet delivery, FWIW immediate
mode was actually the default prior libpcap-1.5.0 (CentOS 6) regardless
of whether a buffer was requested.

The lack of any kind of timeout on the pcap buffer messed with the
libvirt TCK test suite which, even with a generous timeout in place,
timeouts every single time simply because it takes a while until
guest actually starts producing any kind of traffic to fill up
the buffer in place (apart from the DHCP traffic which happens fairly
early on).

Signed-off-by: Erik Skultety <eskultet@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>