ntp: Replace the workaround from 98e34e8e2557 with a patch from upstream

98e34e8e2557 circumvented an upstream patch which caused an IPv6
pool regresson. This patch removes the circumvention and replaces
it with an upstream patch planned for the new release of ntp.

(cherry picked from commit bc02e655872021595c434850fbcbdb8dd11d4a46)