Make xfrm linux-only

The xfrm framework is linux-only. Only implement the respective types
for GOOS=linux to avoid dependencies to x/sys/unix on non-linux or
non-unix platforms. Provide dummy XfrmPolicy and XfrmState types for the
globally defined XfrmPolicy* and XfrmState* functions.

xfrm_monitor_linux_test.go

@@ -0,0 +1,52 @@
+package netlink
+
+import (
+	"os"
+	"testing"
+
+	"github.com/vishvananda/netlink/nl"
+)
+
+func TestXfrmMonitorExpire(t *testing.T) {
+	if os.Getenv("CI") == "true" {
+		t.Skipf("Flaky in CI: Intermittently causes 10 minute timeout")
+	}
+	defer setUpNetlinkTest(t)()
+
+	ch := make(chan XfrmMsg)
+	done := make(chan struct{})
+	defer close(done)
+	errChan := make(chan error)
+	if err := XfrmMonitor(ch, nil, errChan, nl.XFRM_MSG_EXPIRE); err != nil {
+		t.Fatal(err)
+	}
+
+	// Program state with limits
+	state := getBaseState()
+	state.Limits.TimeHard = 2
+	state.Limits.TimeSoft = 1
+	if err := XfrmStateAdd(state); err != nil {
+		t.Fatal(err)
+	}
+
+	hardFound := false
+	softFound := false
+
+	msg := (<-ch).(*XfrmMsgExpire)
+	if msg.XfrmState.Spi != state.Spi {
+		t.Fatal("Received unexpected msg, spi does not match")
+	}
+	hardFound = msg.Hard || hardFound
+	softFound = !msg.Hard || softFound
+
+	msg = (<-ch).(*XfrmMsgExpire)
+	if msg.XfrmState.Spi != state.Spi {
+		t.Fatal("Received unexpected msg, spi does not match")
+	}
+	hardFound = msg.Hard || hardFound
+	softFound = !msg.Hard || softFound
+
+	if !hardFound || !softFound {
+		t.Fatal("Missing expire msg: hard found:", hardFound, "soft found:", softFound)
+	}
+}