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

Here is the test result after conversion.

]# ./test_vxlan_vnifiltering.sh
    TEST: Create traditional vxlan device                               [ OK ]
    TEST: Cannot create vnifilter device without external flag          [ OK ]
    TEST: Creating external vxlan device with vnifilter flag            [ OK ]
...
    TEST: VM connectivity over traditional vxlan (ipv6 default rdst)    [ OK ]
    TEST: VM connectivity over metadata nonfiltering vxlan (ipv4 default rdst)  [ OK ]

Tests passed:  27
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>