- Randomize spinlock exponential backoff value, which reduces the chance of

- Randomize spinlock exponential backoff value, which reduces the chance of
  serious spinlock contention (probably) caused by same backoff steps
- Ktr spinlock backoff value and backoff failure
- Under debug sysctl tree, add sysctl node for spinlock backoff limit
- Break long lines

Reviewed-by: dillon@