netdev-linux: Optimize removing policing from an interface.

netdev-linux: Optimize removing policing from an interface.

It is very expensive to start a subprocess and, especially, to wait for it
to complete.  This replaces the most common subprocess operation in
netdev_linux_set_policing() by a Netlink socket operation, which is much
faster.

Without this and the other netdev-linux commits, my 1000-interface test
case runs in 1 min 48 s.  With them, it runs in 25 seconds.