kernel - Improve concurrency in devfs VOPs

kernel - Improve concurrency in devfs VOPs

* Use LK_SHARED instead of LK_EXCLUSIVE whenever possible.  This
  significantly reduces lock congestion for getattr(), read(),
  and readlink().

* Check the new D_QUICK flag and, if set, devfs can avoid lock
  congestion for open() and close() on devices (e.g. such as on
  /dev/null, /dev/urandom, etc).