netlink: clear IPv6 embedded scope when dumping route gateways.

Reported by: zarychtam@plan-b.pwste.edu.pl
MFC after: 3 days
Approved by: re(cperciva)

(cherry picked from commit b9b2184322da85e1f4d52e3b0a6322dd214210ec)
(cherry picked from commit d101991d8dcc1e056d1c06ca83b88826f4d65ae5)