iproute2: Ignore EADDRNOTAVAIL errors during address flush operation

iproute2: Ignore EADDRNOTAVAIL errors during address flush operation

I found recently that, if I disabled address promotion in the kernel, that
ip addr flush dev <dev>

would fail with an EADDRNOTAVAIL errno (though the flush operation would in fact
flush all addresses from an interface properly)

Whats happening is that, if I add a primary and multiple secondary addresses to
an interface, the flush operation first ennumerates them all with a GETADDR |
DUMP operation, then sends a delete request for each address.  But the kernel,
having promotion disabled, deletes all secondary addresses when the primary is
removed.  That means, that several delete requests may still be pending in the
netlink request for addresses that have been removed on our behalf, resulting in
EADDRNOTAVAIL return codes.

It seems the simplest thing to do is to understand that EADDRUNAVAIL isn't a
fatal outcome on a flush operation, as it just indicates that an address which
you want to remove is already removed, so it can safely be ignored.

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
CC: Stephen Hemminger <stephen@networkplumber.org>
CC: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>