Using NLM_F_ACK in these functions is confusing because they don't
parse any netlink response.

In fact, NLM_F_ACK is only required internally by rtnl_talk(), which
already adds it when its 'answer' parameter is NULL. Therefore it's
useless to manually set it in functions that don't set 'answer'.
Signed-off-by: NGuillaume Nault <g.nault@alphalink.fr>