Make some adjustments to the buffer cache:

Make some adjustments to the buffer cache:

* Retain B_ERROR instead of clearing it.

* Change B_ERROR's behavior.  It no longer causes the buffer to be
  invalidated on write.

* Change B_NOCACHE's behavior.  It no longer causes the buffer to be
  invalidated while the buffer is marked dirty.

* Code that was supposed to re-dirty a failed write buffer in brelse()
  was not running because biodone() cleared the fields brelse() was
  testing.  Move the code to biodone().

* When attempting to reflush B_DELWRI|B_ERROR'd buffers, sleep a tick
  to try to avoid a live-lock.