selftests: forwarding: Use same VRF for port and VLAN upper

The test creates a separate VRF for the VLAN upper, but does not destroy
it during cleanup, resulting in "RTNETLINK answers: File exists" errors.

Fix by using the same VRF for the port and its VLAN upper. This is OK
since their IP addresses do not overlap.

Before:

 # ./bridge_locked_port.sh
 TEST: Locked port ipv4                                              [ OK ]
 TEST: Locked port ipv6                                              [ OK ]
 TEST: Locked port vlan                                              [ OK ]

 # ./bridge_locked_port.sh
 RTNETLINK answers: File exists
 RTNETLINK answers: File exists
 RTNETLINK answers: File exists
 RTNETLINK answers: File exists
 RTNETLINK answers: File exists
 RTNETLINK answers: File exists
 TEST: Locked port ipv4                                              [ OK ]
 TEST: Locked port ipv6                                              [ OK ]
 TEST: Locked port vlan                                              [ OK ]

After:

 # ./bridge_locked_port.sh
 TEST: Locked port ipv4                                              [ OK ]
 TEST: Locked port ipv6                                              [ OK ]
 TEST: Locked port vlan                                              [ OK ]

 # ./bridge_locked_port.sh
 TEST: Locked port ipv4                                              [ OK ]
 TEST: Locked port ipv6                                              [ OK ]
 TEST: Locked port vlan                                              [ OK ]

Fixes: b2b681a4 ("selftests: forwarding: tests of locked port feature")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>