route/addr: only sent IFA_FLAGS when needed to workaround picky older kernels

Older kernels don't accept receiving unknown netlink attributes.
See net/core/rtnetlink.c, rtnetlink_rcv_msg(). This was fixed by kernel
commit 661d2967b3f1b34eeaa7e212e7b9bbe8ee072b59.

As a workaround, only set the additional attributes, when the user
provided flags that makes this necessary and useful.

https://github.com/thom311/libnl/issues/56
https://bugzilla.redhat.com/show_bug.cgi?id=1063885

Based-on-patch-by: Pavel Kankovsky <peak@argo.troja.mff.cuni.cz>
Acked-by: Thomas Graf <tgraf@suug.ch>
Signed-off-by: Thomas Haller <thaller@redhat.com>