route: sort by priority to identify the default one

[jibi]

In lookupDefaultRoute(), to determine the default route for a given IP
family we list all routes filtering just by the 0.0.0.0 destination.

On some EKS setups where multilple NICs are attached to an instance,
this is not enough to identify a single route, since each interface has
its own default route.

The following example illustrates the problem:

$ ks exec ds/cilium -- ip route list
default via 10.100.1.1 dev eth0
default via 10.100.255.65 dev eth1 metric 10001
169.254.169.254 dev eth0

This will cause the agent to report a fatal error, as it can't find a
single gateway:

cilium-agent level=fatal msg="Error while creating daemon" error="unable to install ip rule for ENI multi-node NodePort: failed to find interface with default route: Found default routes with different netdev ifindices: 2 vs 3" subsys=daemon

To address this issue, this commit changes the logic of
lookupDefaultRoute() to pick the one with the lowest priority.

[maintainer-s-little-helper]

Job 'Cilium-PR-K8s-1.23-kernel-net-next' failed and has not been observed before, so may be related to your PR:

Click to show.

Test Name

K8sHealthTest cilium-health

Failure Output

FAIL: failed to ensure kubectl version: failed to run kubectl version

If it is a flake, comment /mlh new-flake Cilium-PR-K8s-1.23-kernel-net-next so I can create a new GitHub issue to track it.

[jibi]

/test

Job 'Cilium-PR-K8s-1.23-kernel-net-next' hit: #18566 (97.36% similarity)