selftests: forwarding: lib: Extract interface-init functions

The function simple_if_init() does two things: it creates a VRF, then
moves an interface into this VRF and configures addresses. The latter
comes in handy when adding more interfaces into a VRF later on. The
situation is similar for simple_if_fini().

Therefore split the interface remastering and address de/initialization
logic to a new pair of helpers __simple_if_init() / __simple_if_fini(),
and defer to these helpers from simple_if_init() and simple_if_fini().

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