[IPV6] NDISC: Fix is_router flag setting.

We did not send appropriate IsRouter flag if the forwarding setting is
positive even value.  Let's give 1/0 value to ndisc_send_na().

Also, existing users of ndisc_send_na() give 0/1 to override,
we can omit redundant operation in that function.

Bug hinted by Nicolas Dichtel <nicolas.dichtel@6wind.com>.

Signed-off-by: YOSHIFUJI Hideaki <yoshfuji@linux-ipv6.org>
Signed-off-by: David S. Miller <davem@davemloft.net>