which linux partitions have been mapped to FreeBSD partitions.  Now the offset of
the 'a' partition is extracted from the disklabel and compared with the offset of
each partition found in /sys/block/$disk/$disk[1-9]* to find a match.

If the offset of the 'c' partition in the disklabel is 0, then we add the offset
of the FreeBSD slice to the offset of the 'a' partition before trying to find
a match.  This is to support FreeBSD's relative disklabel.