Improve xfrm net test

In kernel_net_test/xfrm_tunnel_test.XfrmVtiTest

The testcases write packets to the tunnel, then check the count of rx/tx
packets number on the interface.

However, the packets are processed *asynchronously* in kernel.

So, receive the packets before check the rx count

Test: run vts/kernel_net_tests
Bug: 117532237
Signed-off-by: Xu YiPing <xuyiping@hisilicon.com>