bgpd: split nexthop-local unchanged peer subgroup

bgpd: split nexthop-local unchanged peer subgroup

5bb99ccad2 ("bgpd: reset ipv6 invalid link-local nexthop") now resets
the link-local when originating and destination peers are not on the
same network segment. However, it does not work all the time.

The fix compares the 'from' and 'peer' global IPv6 address. However,
'peer' refers to one of the peers of subgroup. The subgroup may contain
peers located on different network segment.

Split nexthop-local unchanged peer subgroup by network segment.

Fixes: 5bb99ccad2 ("bgpd: reset ipv6 invalid link-local nexthop")
Signed-off-by: Louis Scalbert <louis.scalbert@6wind.com>