ipv6: expose RFC4191 route preference via rtnetlink

This makes it possible to retain the route preference when RAs are handled in
userspace.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Reviewed-by: Jiri Pirko <jiri@resnulli.us>
Signed-off-by: David S. Miller <davem@davemloft.net>