mlxsw: spectrum_router: Extract a helper to free a RIF

Right now freeing the object that mlxsw uses to keep track of a RIF is as
simple as calling a kfree. But later on as CRIF abstraction is brought in,
it will involve severing the link between CRIF and its RIF as well. Better
to have the logic encapsulated in a helper.

Since a helper is being introduced, make it a full-fledged destructor and
have it validate that the objects tracked at the RIF have been released.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Amit Cohen <amcohen@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>