ipv6: Notify route if replacing currently offloaded one

Similar to the corresponding IPv4 patch, only notify the new route if it
is replacing the currently offloaded one. Meaning, the one pointed to by
'fn->leaf'.

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