[RTNETLINK]: Fix RTNLGRP definitions in rtnetlink.h

I reported a problem and gave hints to the solution, but nobody seemed
to react. So I prepared a patch against 2.6.14.4.

Tested on 2.6.14.4 with "ip monitor addr" and with the program
attached, while adding and removing IPv6 address. Both programs didn't
receive any messages.  Tested 2.6.14.4 + this patch, and both programs
received add and remove messages.

Signed-off-by: Kristian Slavov <kristian.slavov@nomadiclab.com>
Acked-by: Jamal Hadi salim <hadi@cyberus.ca>
ACKed-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>