Use correct partition names for /dev/mapper

Use correct partition names for /dev/mapper

The default configuration of multipath-tools appends "-partN" to
partition nodes. Follow this conventions and do the same.

It fixes for example fdisk -l /dev/mapper/name_of_the_device.

Note that the current implementation only partially fixes the problem. It
does not reflect any udev configuration changes, as udev does not provide
any function to return names of future (or current) partitions of a
particular device. It also does not fix fdisk -l /dev/dm-0.

Signed-off-by: Stanislav Brabec <sbrabec@suse.cz>