Merge branch 'kmem-cache-optimization'

This branch contains kmem cache optimizations designed to resolve
the lockups reported in zfsonlinux/zfs#922.  The lockups were
largely the result of spin lock contention in the slab under low
memory conditions.  Fundamentally, these changes are all designed
to minimize that contention though a variety of methods.

  * Improved vmem cached deadlock detection
  * Track emergency objects in rbtree
  * Optimize spl_kmem_cache_free()
  * Never spin in kmem_cache_alloc()

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
zfsonlinux/zfs#922