mlxsw: spectrum_router: Maintain CRIF for fallback loopback RIF

CRIFs are generally not maintained for loopback RIFs. However, the RIF for
the default VRF is used for offloading of blackhole nexthops. Nexthops
expect to have a valid CRIF. Therefore in this patch, add code to maintain
CRIF for the loopback RIF as well.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Link: https://lore.kernel.org/r/7f2b2fcc98770167ed1254a904c3f7f585ba43f0.1687438411.git.petrm@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>