[kpartx] Don't add 'p' delimiter when you shouldn't

[kpartx] Don't add 'p' delimiter when you shouldn't

The 'p' delimiter is supposed to be added when the base disk name
ends in a digit.  This decision was based on the name given on the
command line, not the canonical device name, so giving /dev/dm-0
instead of /dev/mapper/foo triggered the digit test and added the
'p'.  Changed test to use the canonical name rather than the given
name.