[RTNETLINK]: Fix sending netlink message when replace route.

When you replace route via ip r r command the netlink multicast message is
not send.  This patch corrects it.  NL message is sent with NLM_F_REPLACE
flag.

Addresses http://bugzilla.kernel.org/show_bug.cgi?id=8320

Signed-off-by: Milan Kocian <milon@wq.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: David S. Miller <davem@davemloft.net>