netlink: Get rid of obsolete rtnetlink macros

Removes all RTA_GET*() and RTA_PUT*() variations, as well as the
the unused rtattr_strcmp(). Get rid of rtm_get_table() by moving
it to its only user decnet.

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