[RTNETLINK] Fix & cleanup rtm_min/rtm_max

Converts rtm_min and rtm_max arrays to use c99 designated
initializers for easier insertion of new message families.
RTM_GETMULTICAST and RTM_GETANYCAST did not have the minimal
message size specified which means that the netlink message
was parsed for routing attributes starting from the header.
Adds the proper minimal message sizes for these messages
(netlink header + common rtnetlink header) to fix this issue.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>