dump: preload netfilter modules too

As with the socket diag modules, since we might be using the
ip*filter_tables modules, we should preload those as well, in case the host
system hasn't already loaded them. Really, I should implement netlink
buffer dumping so we can get rid of this hack :)

v2: remember to close /dev/null fd after using it

Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>