kmem slab fixes

kmem slab fixes

- spl_slab_reclaim() 'continue' changed back to 'break' from commit
  37db7d8cf9936e6d2851a4329c11efcd9f61305c.  The original was correct,
  I have added a comment to ensure this does not happen again.
- spl_slab_reclaim() further optimized by moving the destructor call
  in spl_slab_free() outside the skc->skc_lock.  This minimizes the
  length of time the spin lock is held, allows the destructors to
  be invoked concurrently for different objects, and as a bonus makes
  it safe (although unwise) to sleep in the destructors.