kernel - Performance tuning (2)

* Fix but in VOP_CLOSE changes.  All calls to VOP_CLOSE() require the
  vnode to be locked shared or exclusive.  sys_swapoff() wasn't.

Reported-by: swildner
kernel - Fine-grain getnewbuf() and related vfs/bio data structures (2)

* Fix bug where bufinit() was being called prior to SMP initialization,
  so it was only initializing the queues for one cpu instead of all of
  them.
kernel - Performance tuning (3)

* The VOP_CLOSE issues revealed a bigger issue with vn_lock().  Many
  callers do not check the return code for vn_lock() and in nearly all
  of those cases it wouldn't fail anyway due to a prior ref, but it
  creates an API issue.

* Add the LK_FAILRECLAIM flag to vn_lock().  This flag explicitly allows
  vn_lock() to fail if the vnode is undergoing reclamation.

  This fixes numerous issues, particularly when VOP_CLOSE() is called
  during a reclaim due to recent LK_UPGRADE's that we do in some VFS
  *_close() functions.

* Remove some unused LK_ defines.