selftests/xsk: Add missing close() on netns fd

Commit 1034b03e ("selftests: xsk: Simplify cleanup of ifobjects")
removed close on netns fd, which is not correct, so let us restore it.

Fixes: 1034b03e ("selftests: xsk: Simplify cleanup of ifobjects")
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/20220830133905.9945-1-maciej.fijalkowski@intel.com

tools/testing/selftests/bpf/xskxceiver.c

@@ -1606,6 +1606,8 @@ static struct ifobject *ifobject_create(void)
 	if (!ifobj->umem)
 		goto out_umem;
 
+	ifobj->ns_fd = -1;
+
 	return ifobj;
 
 out_umem:
@@ -1617,6 +1619,8 @@ static struct ifobject *ifobject_create(void)
 
 static void ifobject_delete(struct ifobject *ifobj)
 {
+	if (ifobj->ns_fd != -1)
+		close(ifobj->ns_fd);
 	free(ifobj->umem);
 	free(ifobj->xsk_arr);
 	free(ifobj);