[IPV4]: Add fib rule netlink notifications

To really make sense of route notifications in the presence of
multiple tables, userspace also needs to be notified about routing
rule updates.  Notifications are sent to the so far unused
RTNLGRP_NOP1 (now RTNLGRP_RULE) group.

Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>