fix extended partition mapping

fix extended partition mapping

The linux kernel maps the extended partition only
so that LILO can be installed there.  The length is always set
to two sectors to allow this, and most tools know to ignore the
device.  kpartx was mapping the entire extended partition, then
stacking the logical partitions on top of it.  This presented
a device that looked like an entirely separate disk that
contains only the logical partitions.  This patch fixes kpartx
to conform with the normal Linux behavior.