kernel - Fix two NFS crashes

* Fix a bug during unmount when sillyrenames are being terminated.
  When doing a forced unmount, the sillyrename vnode(s) may be VBAD.
  Do not attempt to flush the sillyrename in this case.

* Fix a bug for 'soft' mounts.  Soft failures do not properly set the
  error code which can lead to a NULL pointer dereference in the rpc
  processing code.

  Set the error code to EINTR for soft mounts whos retries have been
  exceeded.
kernel - turn off auto-socket sizing

* Turn off automatic socket sizing for NFS sockets.  Otherwise the socket
  buffer might be reduced to the point where the mbuf interface refuses
  to queue w/EMSGSIZE.

  TODO: We need a better fix.