Filter outbound packets in the kernel

It turns out that under Linux, libpcap implements the SetDirection
filtering in userspace, causing some spurious wake ups of the packet
capture thread.  So set a BPF filter to discard outbound packets inside
the kernel.

pcap.go

@@ -8,9 +8,19 @@ type PcapIO struct {
 	handle *pcap.Handle
 }
 
-func NewPcapIO(ifName string, bufSz int) (pio PacketSourceSink, err error) {
-	pio, err = newPcapIO(ifName, true, 65535, bufSz)
-	return
+func NewPcapIO(ifName string, bufSz int) (PacketSourceSink, error) {
+	pio, err := newPcapIO(ifName, true, 65535, bufSz)
+	if err != nil {
+		return pio, err
+	}
+
+	// Under Linux, libpcap implements the SetDirection filtering
+	// in userspace.  So set a BPF filter to discard outbound
+	// packets inside the kernel.  We do this here rather than in
+	// newPcapIO because libpcap doesn't like this in combination
+	// with a 0 snaplen.
+	err = pio.handle.SetBPFFilter("inbound")
+	return pio, err
 }
 
 func NewPcapO(ifName string) (po PacketSink, err error) {