IPv6: Temp addresses are immediately deleted.

IPv6: Temp addresses are immediately deleted.

There is a bug in the interaction between ipv6_create_tempaddr and
addrconf_verify.  Because ipv6_create_tempaddr uses the cstamp and tstamp
from the public address in creating a private address, if we have not
received a router advertisement in a while, tstamp + temp_valid_lft might be
< now.  If this happens, the new address is created inside
ipv6_create_tempaddr, then the loop within addrconf_verify starts again and
the address is immediately deleted.  We are left with no temporary addresses
on the interface, and no more will be created until the public IP address is
updated.  To avoid this, set the expiry time to be the minimum of the time
left on the public address or the config option PLUS the current age of the
public interface.

Signed-off-by: Glenn Wurster <gwurster@scs.carleton.ca>
Signed-off-by: David S. Miller <davem@davemloft.net>