net/ntp: Another patch to address IPv6 pool regression

43537eb9c3e5 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.

Obtained from: src c1767cf87cb6

(cherry picked from commit c6ccd7ce6e64d7b036ad89d23b9a1ffc78740cb6)