hung_task: check the value of "sysctl_hung_task_timeout_sec"

As sysctl_hung_task_timeout_sec is unsigned long, when this value is
larger then LONG_MAX/HZ, the function schedule_timeout_interruptible in
watchdog will return immediately without sleep and with print :

schedule_timeout: wrong timeout value ffffffffffffff83

and then the funtion watchdog will call schedule_timeout_interruptible
again and again. The screen will be filled with

"schedule_timeout: wrong timeout value ffffffffffffff83"

This patch does some check and correction in sysctl, to let the function
schedule_timeout_interruptible allways get the valid parameter.

Signed-off-by: Liu Hua <[e-mail]>
Tested-by: Satoru Takeuchi <[e-mail]>
Cc: <[e-mail]> [3.4+]
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

Documentation/sysctl/kernel.txt

···
 This file shows up if CONFIG_DETECT_HUNG_TASK is enabled.
 
 0: means infinite timeout - no checking done.
+Possible values to set are in range {0..LONG_MAX/HZ}.
 
 ==============================================================
 

kernel/sysctl.c

···
 static int ngroups_max = NGROUPS_MAX;
 static const int cap_last_cap = CAP_LAST_CAP;
 
+/*this is needed for proc_doulongvec_minmax of sysctl_hung_task_timeout_secs */
+#ifdef CONFIG_DETECT_HUNG_TASK
+static unsigned long hung_task_timeout_max = (LONG_MAX/HZ);
+#endif
+
 #ifdef CONFIG_INOTIFY_USER
 #include <linux/inotify.h>
 #endif
···
 		.maxlen		= sizeof(unsigned long),
 		.mode		= 0644,
 		.proc_handler	= proc_dohung_task_timeout_secs,
+		.extra2		= &hung_task_timeout_max,
 	},
 	{
 		.procname	= "hung_task_warnings",