[IPV6]: Nearly complete kzalloc cleanup for net/ipv6

Stupidly use kzalloc() instead of kmalloc()/memset()
everywhere where this is possible in net/ipv6/*.c .

Signed-off-by: Ingo Oeser <ioe-lkml@rameria.de>
Signed-off-by: David S. Miller <davem@davemloft.net>