softlockup: Fix hung_task_check_count sysctl

I'm seeing spikes of up to 0.5ms in khungtaskd on a large
machine. To reduce this source of jitter I tried setting
hung_task_check_count to 0:

 # echo 0 > /proc/sys/kernel/hung_task_check_count

which didn't have the intended response. Change to a post
increment of max_count, so a value of 0 means check 0 tasks.

Signed-off-by: Anton Blanchard <anton@samba.org>
Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: msb@google.com
LKML-Reference: <20091127022820.GU32182@kryten>
Signed-off-by: Ingo Molnar <mingo@elte.hu>