ipfix: Skip BFD packets.

ipfix: Skip BFD packets.

The patch is to skip BFD packets in ipfix.
Bidirectional Forwarding Detection (BFD) packets are for monitoring
the tunnel link status and consumed by ovs itself, no need to sample
them.
Refer to IETF RFC 5881, BFD control packets are the UDP packets with
destination port 3784 and BFD echo packets are the UDP packets with dst
destination port 3785.
Ipfix will skip both BFD control packets and BFD echo packets.

Signed-off-by: Wenyu Zhang <wenyuz@vmware.com>
[blp@ovn.org added check for IP]
Signed-off-by: Ben Pfaff <blp@ovn.org>