selftests: net: Add VXLAN MDB test

Add test cases for VXLAN MDB, testing the control and data paths. Two
different sets of namespaces (i.e., ns{1,2}_v4 and ns{1,2}_v6) are used
in order to test VXLAN MDB with both IPv4 and IPv6 underlays,
respectively.

Example truncated output:

 # ./test_vxlan_mdb.sh
 [...]
 Tests passed: 620
 Tests failed:   0

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Nikolay Aleksandrov <razor@blackwall.org>
Signed-off-by: David S. Miller <davem@davemloft.net>