ofproto-dpif-upcall: Expire netflow flow when revalidate_ukey failed

This fixes missing netflow flows in
"ofproto-dpif - NetFlow flow expiration" tests.

Acked-by: Joe Stringer <joestringer@nicira.com>
Signed-off-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>

Conflicts:
	ofproto/ofproto-dpif-upcall.c