Fix flaky netlink and overlay cleanup tests

[chruffins]

summary

Fixes two flaky Linux cleanup tests found by the test-agent:

• retry interrupted netlink link dumps during orphaned TAP and traffic-control cleanup
• use the existing bounded deletion helper in TestOverlayDiskCleanupOnDelete so the test tolerates hypervisor teardown completing after the first delete attempt

testing

• go test ./lib/network
• TestCreateInstanceWithNetwork passed 5 times
• TestOverlayDiskCleanupOnDelete passed 5 times
• full no-cache Linux suite passed 3 consecutive times (269s, 488s, 421s)