ipv6: Add fib6_nh_init and release to stubs

Add fib6_nh_init and fib6_nh_release to ipv6_stubs. If fib6_nh_init fails,
callers should not invoke fib6_nh_release, so there is no reason to have
a dummy stub for the IPv6 is not enabled case.

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