ntp: fix CVE-2013-5211

The monlist feature in ntp_request.c in ntpd in NTP before
4.2.7p26 allows remote attackers to cause a denial of service
(traffic amplification) via forged (1) REQ_MON_GETLIST or
(2) REQ_MON_GETLIST_1 requests, as exploited in the wild
in December 2013.

Signed-off-by: Zhang Xiao <xiao.zhang@windriver.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>