ifupdown: do not fail if interface disappears during ifdown

ifupdown: do not fail if interface disappears during ifdown

Interface may not exist because it got deleted by an ifdown hook script
earlier. This may happen when a virtual interface, such as VLAN, has multiple
iface blocks defined.

function                                             old     new   delta
static_down6                                          14      40     +26
static_down                                           54      70     +16
------------------------------------------------------------------------------
(add/remove: 0/0 grow/shrink: 2/0 up/down: 42/0)               Total: 42 bytes

Signed-off-by: Kaarle Ritvanen <kaarle.ritvanen@datakunkku.fi>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>