kernel/hung_task.c: convert simple_strtoul to kstrtouint

sysctl_hung_task_panic has been changed to unsigned int.  use kstrtouint
instead of obsolete simple_strtoul

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Cc: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>