selftests: test_vxlan_under_vrf: Fix broken test case

selftests: test_vxlan_under_vrf: Fix broken test case

The purpose of the last test case is to test VXLAN encapsulation and
decapsulation when the underlay lookup takes place in a non-default VRF.
This is achieved by enslaving the physical device of the tunnel to a
VRF.

The binding of the VXLAN UDP socket to the VRF happens when the VXLAN
device itself is opened, not when its physical device is opened. This
was also mentioned in the cited commit ("tests that moving the underlay
from a VRF to another works when down/up the VXLAN interface"), but the
test did something else.

Fix it by reopening the VXLAN device instead of its physical device.

Before:

 # ./test_vxlan_under_vrf.sh
 Checking HV connectivity                                           [ OK ]
 Check VM connectivity through VXLAN (underlay in the default VRF)  [ OK ]
 Check VM connectivity through VXLAN (underlay in a VRF)            [FAIL]

After:

 # ./test_vxlan_under_vrf.sh
 Checking HV connectivity                                           [ OK ]
 Check VM connectivity through VXLAN (underlay in the default VRF)  [ OK ]
 Check VM connectivity through VXLAN (underlay in a VRF)            [ OK ]

Fixes: 972341597ee9 ("test/net: Add script for VXLAN underlay in a VRF")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20220324200514.1638326-1-idosch@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>