kernel - Fix spinlock bug introduced with windowing

kernel - Fix spinlock bug introduced with windowing

* The exclusive spinlock contention code was improperly assuming that
  non-zero EXCLWAIT bits prevented the SHARED bit from being set.  This
  is no longer true, shared locks can sometimes override EXCLWAIT.
  This assumption could result in spin_lock() returning with a shared
  lock instead of an exclusive lock.

* Fixed by ensuring that the SHARED bit is cleared when resolving the
  contended exclusive lock.

* Should hopefully fix the pmap pv == NULL assertion.

Reported-by: dillon, zrj