kernel - Refactor lockmgr()

kernel - Refactor lockmgr()

* Seriously refactor lockmgr() so we can use atomic_fetchadd_*() for
  shared locks and reduce unnecessary atomic ops and atomic op loops.

  The main win here is being able to use atomic_fetchadd_*() when
  acquiring and releasing shared locks.  A simple fstat() loop (which
  utilizes a LK_SHARED lockmgr lock on the vnode) improves from 191ns
  to around 110ns per loop with 32 concurrent threads (on a 16-core/
  32-thread xeon).

* To accomplish this, the 32-bit lk_count field becomes 64-bits.  The
  shared count is separated into the high 32-bits, allowing it to be
  manipulated for both blocking shared requests and the shared lock
  count field.  The low count bits are used for exclusive locks.
  Control bits are adjusted to manage lockmgr features.

  LKC_SHARED	Indicates shared lock count is active, else excl lock
		count.  Can predispose the lock when the related count
		is 0 (does not have to be cleared, for example).

  LKC_UPREQ	Queued upgrade request.  Automatically granted by
		releasing entity (UPREQ -> ~SHARED|1).

  LKC_EXREQ	Queued exclusive request (only when lock held shared).
		Automatically granted by releasing entity
		(EXREQ -> ~SHARED|1).

  LKC_EXREQ2	Aggregated exclusive request.  When EXREQ cannot be
		obtained due to the lock being held exclusively or
		EXREQ already being queued, EXREQ2 is flagged for
		wakeup/retries.

  LKC_CANCEL	Cancel API support

  LKC_SMASK	Shared lock count mask (LKC_SCOUNT increments).

  LKC_XMASK	Exclusive lock count mask (+1 increments)

  The 'no lock' condition occurs when LKC_XMASK is 0 and LKC_SMASK is
  0, regardless of the state of LKC_SHARED.

* Lockmgr still supports exclusive priority over shared locks.  The
  semantics have slightly changed.  The priority mechanism only applies
  to the EXREQ holder.  Once an exclusive lock is obtained, any blocking
  shared or exclusive locks will have equal priority until the exclusive
  lock is released.  Once released, shared locks can squeeze in, but
  then the next pending exclusive lock will assert its priority over
  any new shared locks when it wakes up and loops.

  This isn't quite what I wanted, but it seems to work quite well.  I
  had to make a trade-off in the EXREQ lock-grant mechanism to improve
  performance.

* In addition, we use atomic_fcmpset_long() instead of
  atomic_cmpset_long() to reduce cache line flip flopping at least
  a little.

* Remove lockcount() and lockcountnb(), which tried to count lock refs.
  Replace with lockinuse(), which simply tells the caller whether the
  lock is referenced or not.

* Expand some of the copyright notices (years and authors) for major
  rewrites.  Really there are a lot more and I have to pay more attention
  to adjustments.