ipv6: Check dest prefix length on original route not copied one in rt6_alloc_cow().

After commit 8e2ec639

 ("ipv6: don't
use inetpeer to store metrics for routes.") the test in rt6_alloc_cow()
for setting the ANYCAST flag is now wrong.

'rt' will always now have a plen of 128, because it is set explicitly
to 128 by ip6_rt_copy.

So to restore the semantics of the test, check the destination prefix
length of 'ort'.

Signed-off-by: David S. Miller <davem@davemloft.net>