[UI] Prevent duplicate netmask entries for external access

@tigran.papikyan I tested again; it is working fine with different configurations. Thank you!

Test with this new FB> https://github.com/percona/everest/actions/runs/14908181464

@tigran.papikyan I checked, and it still allows "Save" even when there is an issue with this.

Here is a video:

@Edith Erika Puclla Pareja https://github.com/percona/everest/actions/runs/14908181464

Tigran will confirm the FB for this on Monday