vxlan: mdb: Add MDB get support

Implement support for MDB get operation by looking up a matching MDB
entry, allocating the skb according to the entry's size and then filling
in the response.

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