[kpartx] bug fixes for dos extended partition [2/4]

[kpartx] bug fixes for dos extended partition [2/4]

kpartx usually doesn't create a mapping for extended partition.
However, it does if it's empty.
It's harmless but not consistent.
Attached patch makes it consistent by not creating a mapping
even when the extended partition is empty.

Please consider to apply.

If you have logical partitions in extended partition:
# parted /dev/sdc p
Disk geometry for /dev/sdc: 0.000-34686.000 megabytes
Disk label type: msdos
Minor    Start       End     Type      Filesystem  Flags
1          7.844     31.376  extended
5          7.875     23.532  logical

No mapping is created for extended partition:
# dmsetup table|grep sdc|sort
  sdc5: 0 32067 linear 8:32 16128

However, if the extended partition is empty:
# parted /dev/sdc p
Disk geometry for /dev/sdc: 0.000-34686.000 megabytes
Disk label type: msdos
Minor    Start       End     Type      Filesystem  Flags
1          7.844     31.376  extended

The following mapping is created:
# dmsetup table|grep sdc|sort
  sdc1: 0 48195 linear 8:32 16065

Jun'ichi Nomura, NEC