ipv6: Add neighbor helpers that use the ipv6 stub

Add ipv6 helpers to handle ndisc references via the stub. Update
bpf_ipv6_fib_lookup to use __ipv6_neigh_lookup_noref_stub instead of
the open code ___neigh_lookup_noref with the stub.

Signed-off-by: David Ahern <dsahern@gmail.com>
Reviewed-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>