ifupdown: when flushing addresses with "ip", add label %label%

ifupdown: when flushing addresses with "ip", add label %label%

User report:

or our board we setup eth0:0 on a 10.10.10.x/29 netwrok.

The problem is ip addr flush dev eth0:0 removes all ip addresses from
eth0.  You can see this if you run
ip -stat -stat addr flush dev eth0:0

2: eth0    inet 172.27.105.10/22 brd 172.27.107.255 scope global eth0
       valid_lft forever preferred_lft forever
2: eth0    inet 10.10.10.9/29 scope global eth0:0
       valid_lft forever preferred_lft forever
2: eth0    inet6 fe80::a2f6:fdff:fe18:2b13/64 scope link
       valid_lft forever preferred_lft forever

*** Round 1, deleting 3 addresses ***
*** Flush is complete after 1 round ***

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>