kernel: pipes: Add spin lock/unlock barrier pair

Adds a spin lock/unlock barrier pair after a pipe thread wakes.
After the list of waiting threads is generated, it is possible for
threads on that list to timeout and be removed from the wait queue.
However, since that list was generated before the timeout occurred,
the timed-out thread must wait until the copying is done (the
pipe's spin-lock has been released).

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>