net ipv6: Remove rebundant rt6i_nsiblings initialization

Seting rt->rt6i_nsiblings to zero is rebundant, because above memset
zeroed the rest of rt excluding the first dst memember.

Signed-off-by: Fan Du <fan.du@windriver.com>
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Acked-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Cc: Hideaki YOSHIFUJI <yoshfuji@linux-ipv6.org>
Cc: James Morris <jmorris@namei.org>
Signed-off-by: David S. Miller <davem@davemloft.net>