in_ifadown() was only cleaning up the route table on the originating cpu,

in_ifadown() was only cleaning up the route table on the originating cpu,
it needs to clean up the route table on ALL cpus.  This routine is
called when an interface is brought down, typically during a reinitialization
of the interface's IP address(es).  An example of this would be if
DHCP were re-run on an interface by killing dhclient and re-running it.

Change in_ifadown() to iterate over all cpu's route tables.

Reported-by: multiple people