linux-user: update netlink route types

Add RTA_PREF and RTA_CACHEINFO.

Fix following errors when we start gedit:

  Unknown host RTA type: 12
  Unknown host RTA type: 20

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <20180820171557.7734-4-laurent@vivier.eu>