netlink: fix IPv6 route addition with link-local gateway

netlink: fix IPv6 route addition with link-local gateway

Currently kernel assumes that IPv6 gateway address is in "embedded"
 form - that is, for the link-local IPv6 addresses, interface index
 is embedded in bytes 2 and 3 of the address.
Fix address embedding in netlink by wrapping nhop_set_gw() in the
 netlink-specific nl_set_nexthop_gw(), which does such embedding
 automatically.

Reported by:	Marek Zarychta <zarychtam@plan-b.pwste.edu.pl>
MFC after:	3 days

(cherry picked from commit c7c348162101a70b1f99299578dabe1d3b100e48)