neigh: ipv6: respect default values set before an address is assigned to device

Make the behaviour similar to ipv4. This will allow user to set sysctl
default neigh param values and these values will be respected even by
devices registered before (that ones what do not have address set yet).

Signed-off-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>