tools/selftests: Add a VXLAN+IPsec traffic test

tools/selftests: Add a VXLAN+IPsec traffic test

There are VXLAN tests and IPsec tests, but there is no test that
combines the two protocols and exercises the tunnel-over-ipsec code
paths. Fix that by adding a traffic test with VXLAN and IPsec using
crypto offload. This is runnable on HW which supports ESP offload (so no
nsim unfortunately).

Traffic is done with iperf3 and the test validates that there are no
packet drops and iperf3 can get to at least 100 Mbps (a very
conservative value on today's crypto offload HW, as it can typically
reach multi-Gbps rates).

Ran right now, the test fails due to a recently exposed bug in xfrm,
which will be fixed in the next patch:
 # ./tools/testing/selftests/drivers/net/hw/ipsec_vxlan.py
 TAP version 13
 1..4
 # Check| At ./tools/testing/selftests/drivers/net/hw/ipsec_vxlan.py,
 # line 161, in test_vxlan_ipsec_crypto_offload:
 # Check|     ksft_eq(drops_after - drops_before, 0,
 # Check failed 189 != 0 TX drops during VXLAN+IPsec
 # Check| At ./tools/testing/selftests/drivers/net/hw/ipsec_vxlan.py,
 # line 163, in test_vxlan_ipsec_crypto_offload:
 # Check|     ksft_ge(bw_gbps, 0.1,
 # Check failed 0.0015058278404812596 < 0.1 Minimum 100Mbps over
 # VXLAN+IPsec
 not ok 1 ipsec_vxlan.test_vxlan_ipsec_crypto_offload.outer_v4_inner_v4
 ...

Signed-off-by: Cosmin Ratiu <cratiu@nvidia.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>