• Fix calculation of data slice size when NANO_DATASIZE=-1.
  
  Due to a typo, setting NANO_DATASIZE=-1 resulted in the data slice being the same size as entire image instead of the size of the remaining space on the image.

• Fix detection of overcommit of the slices.
  
  This fix mainly result in a nicer error than when newfs etc. tries to write beyond the end of the disk image.

MFC after: 2 weeks
X-MFC after: RELENG_7 is open again