[IPV6]: Fix Temporary Address Generation

From: Hiroyuki YAMAMORI <h-yamamo@db3.so-net.ne.jp>

Since regen_count is stored in the public address, we need to reset it
when we start renewing temporary address.

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