NFS - Properly handle NFSv3 EOF short-reads

NFS - Properly handle NFSv3 EOF short-reads

* Short reads were not causing the remainder of the buffer to be
  zero'd out in all cases, causing the fsx filesystem test to fail.

* An EOF condition was sometimes improperly reporting a non-zero
  bp->b_resid.  Race against the server where the client's idea of
  the file size differs from the server could result in a non-zero
  bp->b_resid after a BIO.

  Zero out the remaining space and and set bp->b_resid to 0.  The
  NFS client code always specifies buffers within the bounds of
  the file.

Reported-by: Antonio Huete Jimenez <tuxillo@quantumachine.net>