NFS calculation of max commit size

[Rick Macklem]

Just to correct myself...

- The NFS VOP_WRITE() can keep track of a running total of how many
  bytes is being written:
  - add uio_resid to this running total at the beginning of the VOP_WRITE()
    and subtract it back out at the end of VOP_WRITE().
This was incorrectly stated. The value should be subtracted back out when
the write rpc completes (ie. buffer has been flushed), since the running
total needs to be "how many unwritten NFS bytes are in the buffer cache".
At least that was what I was/am thinking...

rick