ntp: Another patch to address IPv6 pool regression

98e34e8e2557 circumvented an upstream patch which caused an IPv6
pool regression. This patch, discussed in
https://bugs.ntp.org/show_bug.cgi?id=3958, addresses another
unworkable combination of link-local local address with
non-link-local server.

(cherry picked from commit c1767cf87cb64c25426fd7fe119be283b134509a)