Fix cache flush ordering on NTA expiry

Fix cache flush ordering on NTA expiry

dns_view_flushnode() was called in the delete_expired() async
callback, which runs after the query that detected the NTA expiry.
This created a race: the query would proceed with stale cached data
from the NTA period before the flush had a chance to run, resulting
in transient SERVFAIL with EDE 22 (No Reachable Authority).

Move dns_view_flushnode() into dns_ntatable_covered() so the cache
is flushed synchronously when the expiry is detected, before the
query continues.

Also simplify the expiry comparison in delete_expired() to a direct
pointer comparison (nta == pval) instead of comparing expiry
timestamps.