hung_task: allow hung_task_panic when hung_task_warnings is 0

hung_task: allow hung_task_panic when hung_task_warnings is 0

Previously hung_task_panic would not be respected if enabled after
hung_task_warnings had already been decremented to 0.

Permit the kernel to panic if hung_task_panic is enabled after
hung_task_warnings has already been decremented to 0 and another task
hangs for hung_task_timeout_secs seconds.

Check if hung_task_panic is enabled so we don't return prematurely, and
check if hung_task_warnings is non-zero so we don't print the warning
unnecessarily.

[[email protected]: fix off-by-one]
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: John Siddle <[email protected]>
Cc: Tetsuo Handa <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>