ipv6: Handle multipath route deletion notification

When an entire multipath route is deleted, only emit a notification if
it is the first route in the node. Emit a replace notification in case
the last sibling is followed by another route. Otherwise, emit a delete
notification.

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>