net: route: Fix retrieving route nexthop

net_ipv6_nbr_lookup_by_index() will always find first matching entry
in IPv6 nbr table with lladdr index. But there can be multiple neighbors
linked to same lladdr index. So find route nexthop with ipv6_nbr_data
from nbr(ipv6 nbr) data pointer.

Change-Id: I5081d40330f5bc1ef0d96def03f4add4808b2fe9
Signed-off-by: Ravi kumar Veeramally <ravikumar.veeramally@linux.intel.com>