selftests/net: convert test_vxlan_mdb.sh to run it in unique namespace

JIRA: https://issues.redhat.com/browse/RHEL-77173

commit a8258e6
Author: Hangbin Liu <liuhangbin@gmail.com>
Date:   Wed Dec 6 15:07:55 2023 +0800

    selftests/net: convert test_vxlan_mdb.sh to run it in unique namespace

    Here is the test result after conversion.

    ]# ./test_vxlan_mdb.sh

    Control path: Basic (*, G) operations - IPv4 overlay / IPv4 underlay
    --------------------------------------------------------------------
    TEST: MDB entry addition                                            [ OK ]

    ...

    Data path: MDB torture test - IPv6 overlay / IPv6 underlay
    ----------------------------------------------------------
    TEST: Torture test                                                  [ OK ]

    Tests passed: 620
    Tests failed:   0

    Acked-by: David Ahern <dsahern@kernel.org>
    Signed-off-by: Hangbin Liu <liuhangbin@gmail.com>
    Reviewed-by: Ido Schimmel <idosch@nvidia.com>
    Tested-by: Ido Schimmel <idosch@nvidia.com>
    Signed-off-by: David S. Miller <davem@davemloft.net>

Signed-off-by: CKI Backport Bot <cki-ci-bot+cki-gitlab-backport-bot@redhat.com>

Author: CKI Backport Bot