xen/netloop: make netloop permanent

xen/netloop: make netloop permanent

with reference to RH BZ#567540 [0], this patch makes the netloop
module permanent (like netback is currently). It reverts parts of
xen-unstable c/s 9019:271cb04a4f2b [1] [2] (though that has a typo:
"__init clean_loopback", so it was probably changed later too).

The patch fixes the problem of "rmmod netloop" hanging, resulting in
blocked tasks and inability to shut down cleanly:

... kernel: unregister_netdevice: waiting for veth4 to become
free. Usage count = 1

The problem was also reported for Debian [3] and on the Fedora-xen
mailing list [4].

[0] https://bugzilla.redhat.com/show_bug.cgi?id=567540
[1] http://lists.xensource.com/archives/html/xen-devel/2006-02/msg01033.html
[2] http://xenbits.xensource.com/xen-unstable.hg?rev/271cb04a4f2b
[3] http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=425703
[4] http://www.redhat.com/archives/fedora-xen/2007-April/msg00074.html

From: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Keir Fraser <keir@xen.org>