ipv6: Don't drop cache route entry unless timer actually expired.

This is ipv6 variant of the commit 5e016cbf.. ("ipv4: Don't drop
redirected route cache entry unless PTMU actually expired")
by Guenter Roeck <guenter.roeck@ericsson.com>.

Remove cache route entry in ipv6_negative_advice() only if
the timer is expired.

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