ipv6: don't set DST_NOCOUNT for remotely added routes

DST_NOCOUNT should only be used if an authorized user adds routes
locally. In case of routes which are added on behalf of router
advertisments this flag must not get used as it allows an unlimited
number of routes getting added remotely.

Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>