net: if: Make sure IPv6 RS timer is not added twice to list

It might happen that if the network interface is going up/down
fast enough, the RS timer could get inserted to the active RS
timer list twice. This would then cause a forever loop in
rs_timeout() when traversing the active list.

Signed-off-by: Jukka Rissanen <jukka.rissanen@nordicsemi.no>