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>