libfdisk: (sgi) generate partname according to partition position

The SGI drive currently counts only usable partitions, that's wrong as
Linux kernel counts all partition to generate partition number. For
example:

Kernel:
	sdb                                             8:16   0   600M  0 disk
	├─sdb9                                          8:25   0     2M  0 part
	└─sdb11                                         8:27   0 596.2M  0 part

Fdisk:
	Pt# Device    Info Start       End   Sectors Id System
	 9: /dev/sdb1          0      4095      4096  0 SGI volhdr
	11: /dev/sdb2          0   1220939   1220940  6 SGI volume

Reported-by: Phillip Susi <psusi@ubuntu.com>
Signed-off-by: Karel Zak <kzak@redhat.com>