Use inbound policy in TestXfrmPolicyWithOptional

Since kernel commit
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=3d776e31c841b
optional tunnel/BEET mode templates in outbound policies are rejected.

Use an inbound policy instead to fix the test failure on newer kernels.

xfrm_policy_test.go

@@ -1,3 +1,4 @@
+//go:build linux
 // +build linux
 
 package netlink
@@ -195,6 +196,7 @@ func TestXfrmPolicyWithOptional(t *testing.T) {
 	defer setUpNetlinkTest(t)()
 
 	pol := getPolicy()
+	pol.Dir = XFRM_DIR_IN
 	pol.Tmpls[0].Optional = 1
 
 	if err := XfrmPolicyAdd(pol); err != nil {