selftests: mlxsw: Test veto of unsupported VXLAN FDBs

mlxsw doesn't implement offloading of all types of FDB entries that the
VXLAN driver supports. Test that such FDB entries are rejected. That
makes sure that the decision made by the existing validation code in
mlxsw propagates up the stack. It also exercises rollback functionality
in VXLAN, and tests that extack is returned.

Signed-off-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>