configure: allow trafgen build when only TPACKET_V2 is available

On machines like RHEL6 that do not have support for TPACKET_V3, we
should still allow at least trafgen to be built for network testing.
Thus, do something useful with the TPACKET_V3 configure check and
comment out in ring.h relevant structures.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>