netlink: validate rtable value in RTM_<NEW|DEL|GET>ROUTE.
Reported by: Stefan Grundmann <sg2342@googlemail.com>
Approved by: re(cperciva)
MFC after: 1 day
(cherry picked from commit f2f7911c5513096e46422ad7756bc90c13c6e6d8)
(cherry picked from commit e9296dc1bcd21cd9d719389cd04235ce4513e84d)