kernel - Change lockmgr LK_SHARED behavior to fix improper recursion return

kernel - Change lockmgr LK_SHARED behavior to fix improper recursion return

* When obtaining a LK_SHARED lock in a situation where you already own the
  lock LK_EXCLUSIVE, lockmgr would downgrade the lock to shared.

  This creates a very serious problem when large procedural recursions get
  a lock that is already being held exclusively but request a shared lock.
  When these recursions return the original top level will find its lock is
  no longer exclusive.

* This problem occured with vnode locks when a VOP_WRITE operation on a mmap'd
  space causes a VM fault which then turns around and issues a read().
  When the fault returns the vnode wound up locked shared instead of
  exclusive.

* Fix the problem by NOT downgrading an exclusive lock to shared when
  recursing on LK_SHARED.  Simply add another count to the exclusive
  lock.