libfdisk: (dos) Fix check error message when CHS calculated sector does not match LBA

libfdisk: (dos) Fix check error message when CHS calculated sector does not match LBA

Function check() checks that LBA addresses sector number matches CHS sector
number. But error message contains information about "previous sectors" and
"total sectors" which is misleading so change it.

Also rename variables 'start' and 'total' to make it clear what value is
stored in them. Note that in 'start' variable is currently stored last LBA
partition sector and in 'total' is stored last CHS partition sector. So
neither sector where partition starts nor total sectors of partition.

Code and check logic looks to be correct, just error message and variable
names are misleading. Therefore no functional changes in this patch.

Signed-off-by: Pali Rohár <pali.rohar@gmail.com>