mlxsw: spectrum_router: Extract a helper for RIF migration

RIF configuration contains a number of parameters that cannot be changed
after the RIF is created. For the IPIP loopbacks, this is currently worked
around by creating a new RIF with the desired configuration changes
applied, and updating next hops to the new RIF, and then destroying the old
RIF. This operation will be useful as a reusable atom, so extract a helper
to that effect.

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