netfilter: xt_IDLETIMER: replace snprintf in show functions with sysfs_emit

coccicheck complains about the use of snprintf() in sysfs show
functions:
WARNING use scnprintf or sprintf

Use sysfs_emit instead of scnprintf, snprintf or sprintf makes more
sense.

Reported-by: Zeal Robot <zealci@zte.com.cn>
Signed-off-by: Jing Yao <yao.jing2@zte.com.cn>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>