IPv6: keep route for tentative address

Recent changes preserve IPv6 address when link goes down (good).
But would cause address to point to dead dst entry (bad).
The simplest fix is to just not delete route if address is
being held for later use.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: David S. Miller <davem@davemloft.net>