ipv6: Use interface max_desync_factor instead of static default

max_desync_factor can be configured per-interface, but nothing is
using the value.

Reported-by: Piotr Lewandowski <piotr.lewandowski@gmail.com>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: David S. Miller <davem@davemloft.net>