mlxsw: spectrum_router: Rollback virtual router adjacency pointer update

In the rare case where the adjacency pointer cannot be updated for a
given virtual router, rollback the operation so that virtual routers
that are already using the new index will use the old one again.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>