Description of problem:

OVN supports FDB learning on localnet ports since https://github.com/ovn-org/ovn/commit/93514df0d4c8fe7986dc5f287d7011f420d1be6d

If this is enabled it may lead to an unbounded ("very large" to be more precise) number of FDB entries to be learnt on the localnet port (usually connected to an external network).

We probably need a mechanism in place to protect OVN against that.  A potential idea is to implement FDB aging (similar to MAC binding - ARP - aging that's available since https://github.com/ovn-org/ovn/commit/1a947dd3073628d2f2655f46ee7d3db62ed15b55).