hung task: check specific tasks for long uninterruptible sleep state

khungtask by default monitors all tasks for long unterruptible sleep.
This change introduces a sysctl option, /proc/sys/kernel/
hung_task_selective_monitoring, to enable monitoring selected tasks.
If this sysctl option is enabled then only the tasks with
/proc/$PID/hang_detection_enabled set are to be monitored,
otherwise all tasks are monitored as default case.

Some tasks may intentionally moves to uninterruptable sleep state,
which shouldn't leads to khungtask panics, as those are recoverable
hungs. So to avoid false hung reports, add an option to select tasks
to be monitored and report/panic them only.
By default, enable the feature always to monitor selected tasks.

Change-Id: I48cd8cfe73dbe2b577541fe9607190eac5556bb2
Signed-off-by: Imran Khan <kimran@codeaurora.org>
Signed-off-by: Lingutla Chandrasekhar <clingutla@codeaurora.org>
[sramana: Resolved minor merge conflict]
Signed-off-by: Srinivas Ramana <sramana@codeaurora.org>